GH,
Not my bowser I'm delighted to say, and I'm afraid I don't know what happened in this particular occasion to allow the vehicle to move. You're absolutely right about the need to immobilise the vehicle.
Something else I forgot to add: NO radio transmissions, and all autonomous emitters (radalt, IFF, DME/TACAN etc) off before bonding. HF radio should also be off, as even receiving messages causes a significant current through aerials.