The film "Riding up" would have been my first guess. But if you are satisfied that you can rule out that possibility, perhaps you are not fully removing the slide when shooting.
Originally Posted by bobwysiwyg
I have a few sheets of 4x5 film from my beginner days when I did just that. For some reason, that was completely logical at the time, I thought it clever to leave the dark slide just barely in the film holder. I was bracketing and not concerned with composition or focusing at that point; just the time needed to wrap up the shot.
Needless to say, I never repeated that mistake.