What kind of laying system are you talking about? Free-range or aviary?

One possibility is that the birds got a fright from something (can be any kind of rodents for example, or an unknown object).

Another possibility for example in aviary systems is the light. Layers like to crowd in the darker spots.

It is best to just watch the birds for a while and look for any suspicious behavior that could relate to the problem.

There must be a stress source in farm. some time nutritional deficiencies cause  aggressive manners in farm. As Attila said over light could cause this problem. you must find where stress comes from.

what is the age of the animals, and what is the breeding system? temperature, light or darkness and nutritional deficiencies. if it occurs suddenly or temporally, then there is and extrinsic source of stress

Are there things common to all these farms? One can think of breed, age of birds, parent stock, feed supplier, vaccinations, veterinarian(?), housing system, feeding system, watering system. lighting regime, time of day or year, vicinity to air fields, motorways, types of industry. These factors might predispose or cause the problems observed.

If there are none found then probably just hope for the best. Some problems will also disappear without doing anything.

farm management need to be checked

I fully agree with the comments made by C.A. Kan. However, if after considering those factors you still fail to identify the exact problem, you could as a last resort minimize the mortalities due to huddling and trampling, by erecting concave barriers at the corners of the pen - i.e. if the birds are on deep litter.

it could also be a temperature problem - maybe the house temperature is too low (cold) for the birds such that they huddle together for warmth crushing the birds in the centre or at the bottom of the pile. 

As mentioned previously - it could also be stress - maybe you have a predator that was able to enter the chicken houses (are the other farms in the same vicinity)? 

This is difficult as you have several farms that are affected - maybe check what is the common denominator in the affected farms ?

Multiple factors! Even a sudden sound like a passing helicopter, a noisy kid,  or an animal in a corner of the shed can cause panic and piling of chicken and consequently suffocation. Based on my experience low temp does not cause this. These were my experiences of the time I was practicing as a vet.

This problem, which is commonly called smothering in the UK, is increasingly reported by farmers and accounts for a substantial proportion of all mortality on farm. Yes, it may occur as a consequence of fearful panic flying but probably more often seems to be a spontaneous behaviour where birds circle around each other and start to duck beneath the bodies of other hens, eventually piling up and suffocating. Nobody knows yet why this occurs although we suspect genetic influences and personally I believe large contrasts in light may be a factor. We would love to investigate the phenomenon scientifically but its very unpredictability is a challenge. Of course birds may also suffocate at the bottom of a deep communal dustbath - here the solution is to restrict the depth of dustbathing substrate (litter, soil etc).

Do the houses have light control or are they curtain-sided? With non-light-tight sheds you can often get problems with light overnight (e.g. full moon or car headlights) and the birds will creep together resulting in smothering.

Are the farms localized in a certain "loud" area? Maybe in the flightline of an airport? The sound could, as mentioned before cause the stress followed by the crowding. Another possible source of stressfull sounds could be from the machinery in the farm, do you have the opportunity to put up a camera in the stable to check if the crowding correlates with a stressfull noise?

the problem with this specific farmer is, he has 2 houses but only in one house he has this problem. This time for the first time he has this problem and nothing had changed in the houses. It is a house 0f 43.000 layers and at the age of 50 weeks about 18% loses by this problem.

A temperature check might be very helpful.

Very high speculation. Did you check for AE? Crawling is tipical for chicks suffering AE, not for adult hens. But,  they could be infected, and it might be the consequence.

This often occurs with effects from light in the night. If you have open sided houses you can see this happen with a full moon night. Also check for possible car headlights from the road affecting one shed and not the other.