Due to  rapid growth and multiplication and remain blooming due to restricted water movement

the reasons are simple, very appropriate water temperature, nutrient concentration, light, photo-period, probably abundant food particle, no predation, other appropriate 'conditions' for rapid growth and spatial aggregation as well

in some cases eutrophication due to sewage discharge, fertilizers and pesticide input can cause blooming, although in a negative way.

High nutrients input or imbalance of N/P ratio is the major factor responsible for eutrophication. The optimum condition (light, water temperature, high nutrient supply etc. ) supports rapid algal growth or blooming especially in static water resource.

in fact Eutrophication and/or water bloom mostly occurs in a lake besides is the process by which a body of water becomes rich in dissolved nutrients through human-created or chemical processes (such as runoff laden with chemical fertilizers used in agriculture). This is often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life. 

hence, biochemical oxygen demand (BOD) is increased due to biodegradation increase of organic matter entered to the water body. Herein increase in population of algae, phytoplanktons and plants containing chloroplast is required to presence of light and CO2. raising temperature leads to biodegradation increase, resulting increase in CO2 release which supports rapid growth and spatial distribution of such populations.

Its a  specific traits of the bloom forming species to exert competitive pressure to grow rapidly among diverse taxon of phytoplankton. Sometimes subtle changes in stoichiometric parameters of the nutrients or the available red-field N/P/Si ratios which may be determinant of the bloom forming species. In some cases it is the r or K selection theory or it is due the absence of counter grazing pressure to arrest the spiked growth rate. Once a bloom initiates it then translates its massive biotic potential to aggravated by spatial distribution within finite geographic regions of the nutrient flux.

Tek's answer sums up the key factors for general blooms. Many waterways may have already exceeded certain guideline values for nutrients but there may be no sign of any blooms. This is because water temperature or light is not favourable for a sustained growth of algae despite the high nutrient levels.

However, as researchers we need to bear in mind that some blooms could be difficult to explain despite a substantial amount of info on algal blooms. The most shocking foreign algal invasion we had in New Zealand has been Didymo (Didymosphenia geminata a freshwater diatom) whose rapid spread still puzzles many of our scientists.

It was suspected that the above alga was spread accidently by a foreign angler. The bloom was first reported in 2004 and the diatom could be spread with a single droplet of water hence there has been stringent central government biosecurity control to ensure anglers clean their gears before and after fishing. Despite such measures, about 150 rivers have been affected.

The strange situation about this alga is that it tends to occupy the cleanest, coldest and the most pristine waters in New Zealand. Most South Island rivers are alpine rivers and this alga has taken heaven in these rivers ignoring rivers with high nutrient levels and lowland rivers including the North Island rivers.

In short, in a case such as the above many of the well established and researched scientific factors appear to fail towards a sensible scientific explanation of the spread and bloom of alga such as Didymo.

THis is  regarding  the   blooming  of fresh water  diatom Didymosphenia geminate in pristine alpine rivers-  the reasons  I presume  are  higher levels of silicate brought through run off from the  upstream and lack of competing  algal species and lack of foraging  animals.

Many papers exist regarding the theory of Didymo outbreak but none appeared to be conclusive. The key factors that have been consistently reported being light, shallow flows, clear water and rock/pebble bed. Some workers attribute climate change as a cause. The most recent paper (http://www.tandfonline.com/doi/abs/10.1080/0269249X.2014.889041#.U3T-L_ldVOI) indicates low SRP (soluble reactive phosphorus below 2 ppb) as the key driver. This means that in order to reduce the outbreak of Didymo the SRP level in water has to be maintained above 2 ppb threshold.

Runoff from agriculture land and sewage discharge lead to increase in organic nutrients in the water. Hence, there's rapid growth of phytoplankton.

Eutrophication is the cause of blooms in freshwaters and in sea it is due to upwelling of nutrients resulting in rapid growth of a particular species which can positively respond to increased nutrients

The terms “algal blooms” or “water blooms” are not well-defined, but usually refers to a rapid increase in algal density, (total algal biomass or of a few dominant species) at the water surface. I think the term water bloom should not be confused with the term “eutrophication” which describes a long term, multi-year process, nor be confused with seasonal increases in algal biomass. Blooms, under the broadest of definitions, can be caused by either rapid growth or spatial aggregation. I am more comfortable with a strictly growth-related definition, but in practical terms, the information necessary to explain a sudden appearance of dense populations of algae may be more difficult than giving it a name.

Causes of a rapid-growth bloom, may be sudden increases in nutrient concentration, either supplied from the watershed by storm-related runoff or internal sources such as the mixing of nutrient-rich hypolimnetic waters, as occurs during strong wind events. Dinoflagellate or Cyanophyte emergence from the sediments also produce blooms.

Wind-generated aggregations are popularly also called blooms. Under certain conditions dense metalimnetic populations of Cyanophytes may rise to the surface, causing dense surface scums.

I agree with you, Robert  Carlson. 

I think Robert Carlson summarized most of reasons. of course not only algal blooming is problem may be some other exotic aquatic plants which may destroyed entire ecosystem.

Most of the answers tackled the reasons behind algal bloom but the question, I think, is something else. The question was not why there is algal bloom but was what is the mechanism of the bloom. Of course the nutrients play a prime role in cell multiplication but what is the trigger that push the cells to be in such high numbers. Bloom of many species do not start by exploding multiplication of floating cells in response to high concentration of specific nutrient but from resting cells in the sediment such as the case of some dinoflagellates. We have to think of certain environmental triggers beside nutrients or other dissolved substances.. .

I agree with Adil. While nutrients are one of the key drivers of blooms, there are other factors such as changes in water temperature, water flow, riparian shades, algal feeders/grazers and water clarity etc. can also be the causes. There is sufficient evidence where high nutrient levels have not triggered the corresponding blooms because of absence or presence of the above factors.

Hi friends as @ Adil has very rightly pointed out the cause behind the mechanism of water bloom. As all the possible reasons described by @ Robert but @ Selva has given other reasons too I also agree to them. Thanks.