Don't worry, butt's are rather devious things.. my theory is that they do their thing at night while you're asleep and not watching.. When you wake up they act all inocent and cute, and when you ask them; they're all like "girl, you crazy, I've always been this jiggly".. Just so underhanded huh!
So do what you like during the day.. your butt is just waiting for it's big chance at night anyway :p