My own experience with this sort of problem is that most of the time it's caused by a tailing loop. With the dry flies did you get knots in the leader? In my case it is caused by breaking (bending) my wrist at the end of the casting stroke.
There are two other 2 ways I've seen your problem; when fishing a heavily weighted fly on a long thin leader, e.g. #4 woolly bugger on a 12' 6X leader, or when I used a braided loop connector and it slipped partially off the line.
I hope this helps.