Yes, it's trapping of the air or more specifically it's the fact that the air will move to those locations. If it's pretty loose in the legs you'll have a problem if you're not careful. Some divers use Gaiters to solve that, others use ankle weights as a temporary crutch until they get comfortable with it.
If you own the suit already, instead of altering it dive it first in say 20 feet of water and see what it does. I am hoping you either have taken a class or dive with another drysuit diver for your first few dives and know how to recover from a feet first ascent which is a good possibility if the legs are too large. Don't add a lot of air to the suit either.
If you have problems with floaty legs and feet then try gaiters and see if they help, they're cheaper than customizing a suit.