What could cause all four windows to go down when the car is not running and is unoccupied?

Hello, This may be a failure of the wireless control module, or even the remote. If the remote has the unlock button held down within a close enough range, the windows may roll down. Since the issue has happened only once, your biggest issue, as you have alluded to, is security, so ensure everything is removed from the vehicle when you exit. Unfortunately, even though the issue already happened, it is very unlikely that even the dealership will be able to find the cause without the issue becoming persistent or predictable. If it does, take that opportunity to bring it in for service.