I had the exact same thing happen to me once but in my Honda Element, freaked the crap out of me because I was trying to park in a busy city section at the time. After turning the car off and on again the problem resolved itself and never happened again. I recall trying to research it (this was a few years ago) and coming up with nothing. I'd be curious to hear what you find, but hopefully that was the first and last time you experience it!