What would cause all four windows and moonroof to open at the same time?

had the keys with me.

In that case i agree with PG in first answer! The unlock key on the key fob had to have been pushed, inadvertently, and held long enough to trigger this function that your car does have, try it and you will see! You were in range of the vehicle for this to happen at that time. Check in your owners manual for more info. Good thing it wasn't raining! I think i would have jumped out of the car too if that just happened out of the blue with no one around!