I have a client who is always having a tear in his Ankara trousers at the lower part of the back crotch after wearing for a while and sometimes in the first wear.

I've tried different crotch length but it's always been the same. This problem doesn't occur with other fabrics he brings forth except Ankara. His hip size is between 46/48 and thigh is 28. 

I will attach some images for clearer explanation. Thank you