It is called "rebellion" my friend!
What I want to know is this: If both parents are tidy people, how do little kids learn to be untidy? One of the great mysteries of the universe, if you ask me.
I am a very organized and pretty clean person but my kids are complete slobs. They are great kids, don't get me wrong... respectful, make good choices, etc. but when it comes to cleaning up after themselves, man forget it! I just assume it is their way of "pushing the envelope." Much better than what alot of their friends do to their parents in addition to the sloppiness.
I should add that mine are all teens ... with little kids, I would credit it to, as Bill Cosby says, "Kids are brain damaged from birth."