I drove an armored car way back in my college days. Most of the time, when a door "mechanically fails", it is because it has been reported as malfunctioning, and the mechanics are ordered not to fix it.
If the truck has a single back door, which is what most people usually see, it has two (or more) locks. One is a lock like a car door lock, the other is a deadbolt.
Money is mostly put in strong canvas or nylon bags, it also has a seal on the enclosed bag. However, money coming from the Treasury or from banks are sometimes put into the equivalent of extra super duty trash bags, that are form fitted, and sealed with seals, of course.
You never stack money or bags of coins next to doors, because you don't want to step over them 50 times a day on your route.
It takes more than just falling out of a truck for a canvas bag to split, but the plastic bags may burst if they would fall out. I'd bet money the driver and or courier reported that lock as failing on their daily log sheet.
Please note, my info is quite old. Things may have changed since then.