Any idea what causes this?

I hate to beat a dead horse, but you should still make 100% sure its not a vacuum leak. I dont know how comfortable you are with this kind of stuff, but if you're looking for an easy way, a can of carb cleaner will find a vacuum leak faster than you can imagine. It's not very high tech, and it can be bad for engine paint, but it will do the trick. The other thing that comes to mind is an improperly adjusted float level in the carb.