I've found with my personal experience that it's best to only have your heat sources on one side, otherwise when the snake feels like it's to hot, it doesn't have any place to go to. I had a BP do that exact same thing when I had heat on both sides of his tank, and when I changed to just one side he stopped, and hasn't done it since. Well occassionally he'll get under his water dish, but that's only when he's getting ready to shed.