there is no nice way to express it except to say it. the trouser is riding up the butt crack. there are three causes 1. the back fork is too tight or 2. the center back seam is too crooked or 3. its both at the same time. shown are the positions of the original trouser seams they are marked A. the cloth marked B. is used when the cause is tightness at the back fork/thigh the fork is then let out. when the trouser has plenty of ease and the problem is still there the angle of the back seam is too crooked . a new seam C. is used to to correct this. the excess cloth here is cut away, that gives the extra room. tailors call this scooping out the seat. line x represents the bottom of the rise. the alteration should not go below this line. except when the rise is too shallow and needs to be deepened.