The term "weed" can be applied to almost any plant, depending on the situation and the perspective of the person referring to a particular plant as being a "weed".  For example, glyphosate-resistant Zea mays could be a weed if it is found growing and competing with glyphosate-resistant Glycine max. 

Crop-weed associations could be due a number of factors: similarities in growth habit, life cycle, management practices, etc. 

Most weeds tend to be herbaceous because this type of plant tends to have the traits of a fast-growing colonizer that can utilize available resources after a disturbance.  Woody species can be "weedy", but this type tends to appear in later stages of succession. 

It is an adaptive strategy followed by weeds to evade control. In agricultural environment, different selection pressures (Ex. Weed management) that are imposed on these weeds result in such evolution. Not all weeds respond the same to the weed control methods, some die and some adapt.

Phenological similarity between crop and weed is one important factor in the development of crop-specific weeds, however sometimes this similarity can be induced by different selective pressures. One such case is in rice, an intense hand-weeding practices followed in Asia resulted in the evolution of a crop-specific weed Echinocloa crus-galli. Similarly, in the case of wheat, Phalaris minor mimics the crop (resembles crop in several growth stages) making it very difficult to control (basically crop mimicry).

 Hema Sudhakar Duddu Good answer..

weeds respond to management practices and are always evolving to escape control measures such as herbicides. in most cases, broad leaved weeds tend to infest and to be more associated with broad leaved crops mainly because the herbicide active ingredient capable killing the BL weeds may also injure the crop.in this case, growers tend to apply maybe more graminicides, resulting in succession by BL weeds... in other instances, certain weeds just naturally fall out of the spectra controlled by herbicides registered for a particular crop. in this case, such tolerant weeds are expected to be dominant over time and to be in constant association with that particular crop.....

in other reported cases, Tillage practices also influence the weed communities in cropping systems... Conventional  tillage involves annual soil inversion, which servers underground propagating structures of  perennial weeds. this promotes dominance of annual weeds which can escape the soil inversion at seed stage... On the other hand, conservation tillage results in dominance of perennial weeds because their underground peranating structures are not disturbed...

A culturally significant example of mimicry is Lolium temulentum (darnel, referred to in the Bible as 'tares' - Matthew 13:24-30). This is a member of the ryegrass family that infests cereal fields and seems to have had its life-cycle and morphology co-selected with wheat and barley during crop domestication. It is so similar to the cereals it infests that, like them, it is almost totally dependent on humans for propagation. Its seedbank exists in granaries rather than the soil. It is almost extinct in countries that practice intensive agriculture. There is also a rich story to be told about darnel's literary significance. More information that you probably need about all this in: Thomas, H., J. Archer and R. Marggraf Turley. 2011. Evolution, Physiology and Phytochemistry of the Psychotoxic Arable Mimic Weed Darnel (Lolium temulentum L). Progress in Botany 72:73-104.

Besides slight variation in weed species composition per location and ecology, it is common to see similar weed species growing in different crop fields. Example, Ageratum conyzoides, Digitaria spp., and host others can be found in cassava, cowpea, groundnut and several other crop fields. it is not the question 'which weed is specific to which crop'. What is more important is, where is the crop grown?. The issue of weed seed rain and seed bank is another important issue to consider. Factors such as dispersal (water, man, other animals, weed, mechanical, explosive) and allelopathtic potential of specific plants against others are significant in determining which one negatively influenced the other. 

"It is an adaptive strategy followed by weeds to evade control."

Hema, I think the way you phrased it, you attribute "agency" to weeds where there isn't. In my view the reason for crop-specific weeds managing to evade control is not part of their "strategy" but a mere consequence of overlapping phenology or physiology.

In general, any unsolicited plant grown in a special field is called as weed. So, with respect to several factors like timing of cultivation, cultivation status, soil type , environmental factors, etc, we must expect to have seen some type of the so called weeds having closely growth, surviving characteristics (e.g. shape, behavior,growth period and maturity) just like to our main crop within the field while other non relevant ones are not capable to survive under such field (for example flooding situation in paddy fields) conditions.  This is who we think the weeds are crop specific.

Are they? I am not sure. Plants in general are agro-climate or location specific. Human interference may change the plant flora of a location including a crop-field. Admixture of weed species with crops species and their spread to adjacent fields is possible. I have been working on different crops in a single location for the last 3 decades. During this period I have witnessed weed-flushes of different species in different years irrespective of the crops grown. When we controlled a particular species, some other species dominated the weed flora. i have also seen different weed species in the same crop grown in different locations or agro-climatic conditions.

I do agree to a certain extent that some weeds may prefer to grow along with some crop species. Parasitic plants are also host-specific.

Crop weed association could occur due to the eco-physiological reasons such as morphological or physiological plasticity and similarities to crop plants (crop mimicry).  

because of their evolutionary background

I with agree Dr. Barbara Sawicka weed spreading depending on the family, genus, species, subspecies . I addition,say also the crops or accompanying plants

The reason is the similarity of the crop and weed plants to the environmental conditions, the phenotype of the plant and the wells, in addition to the allelopathy.

In intercropping, the best combinaion is two crops with most difference in:

morphological (root depth and form - leaves angle and shape- etc)

physiological (C3-C4, different tolerance to herbicide doese or gene motation for restistant), and

phenological (earlier germination due to different dormancy and breaksown pattern; and different flowering physiological need such as vernaliation and lengh day)

ecologic (seed disoertion and seed bank..)

AND in weed-crop competion, the most the time, best combinaion (for weed) is a case with minumum differences in these factors.

Besided, a crop in a familly, may have differenet allelopathic interaction with a weed compare to other fammily members.

Weeds are crop specific mostly because the initial seeds are not that pure at the time of sowing. The impurities of seeds are indicative of existence of similaries between the main crops seeds and the unwanted plant species.

Weeds are crop specific mostly because the crop and weed plants to the environmental conditions, the phenotype of the plant and the wells, in addition to the allelopathy.

Please check it.

Chapter Crop-Associated Weeds

Weeds are those plants growing out of place and time...means all unwanted plants may become weeds only when they grow inside the crop field and during the life cycle of the crop..amaranthus viridis act as weed in wheat crop when it grows out of place and time..otherwise it is either unwanted plant for us or an economic plant if we utilise it for consumption...weeds are specific to the crop because of adaptation with prevailing environment conditions, use of contaminated seeds while sowing, thereby germination of weed seed along with crop seeds and with adapting prevailing situation that plant will make smooth growing adaptation with that crop...similar family, different species of the same genus plants have more specification with the economic crop plants...like capsella (weed) and brassica(mustard crop) belonging to the family brassicaceae .. similarly, wild rice and rice, wild oats and oat, tiger grass and sugarcane etc .. however all weeds may not be crop specific but most of them are existed in association with a crop and it's prevailing ecosystem...

Weeds are much more intelligent and they know how to survive and fulfill their needs for germination, growth and development. Actually weed crop association term clearly defines the weed-crop specificity.

Similarities in environmental environmental needs with the crop plant