I have one that does this. I attribute it to not feeling like he's got a good enough grip on me, as it only happens when he isn't firmly wrapped around me. It's like he's trying to get the traction to squeeze a little bit harder so that he can feel more secure. Especially if I'm moving. That has to be a bit disorienting for the snake, who is going to want to make sure he's hanging on firmly for the ride.