Bus Driving itself

I arrived at Galenstr at 16:54 ready to return to Spd. Westerwaldstr and before i could change the destination, i get a whirring sound and then the bus launches off on its own it sounds like the engine fan or something but its strange, i can put my foot on the brake to stop it but it rolls off on its own. I always set the bus stop brake and open the rear doors to aid with cooling the bus down but then it launches off on its own, also the handbrake and engine were off.

Anyone else had this problem? its quite worrying, i almost ran down a pedestrian (Imagine going for a smoke and you hear a whirring noise, next thing the bus has catapulted forward onto the pavement!)

Don't go for a smoke then, it's bad for your health!