Bin drivers constantly in and out of the lorry. Chances are he jumped out and forgot to put the handbrake on.
One of the issues of bin drivers being on job and knock. Always in a rush to finish.
I have been working on bin wagons for a few years and not one has ever crashed due the drivers rushing to get finished, drivers rarely get out of the cab on the big town rounds as they are all 3 man crewed!
If you open a door in the Dennis wagons when the vehicle is moving even slightly a voice says “Driver apply the handbrake”