Personally I think quitting was going over the top. With transport (especially with agency work) always expect the unexpected, just put it down to a bad day. I know its a bit soul destroying sat in a broken down vehicle on the side of road but you getting paid for it and you haven’t got to sort it out or pay for the repairs.
As a fellow agency driver I know their own drivers come first. You can be replaced and you got a couple of hours extra overtime out of it, so it carn’t be all bad 