Happens every ten seconds, doesn't it? It's called "reverse mode", and it's handy for decks that are on display in a store because it helps prevent "burn-in" on the faceplate. It's a bit distracting in a car, though.
To turn it off, press "Source" until the deck shuts off, then press the preset button "5".
If you have one of the newer Pioneer decks without preset buttons, then you'll have to turn off Reverse Mode in the function menu.