It is, as Unkchunk says, the steering; more specifically, it's constantly correcting to keep the bike under your center of gravity. You're using exactly the same technique of balancing and weight placement as you do when you walk and run. And it has little or nothing to do with frame geometry, either. For the purest demonstration of this, look at unicyclists. It's clear that gyroscopic force isn't at work when you're rolling the wheel back and forth under you to stay upright, much less frame geometry.