By charge I don't mean they literally overrun the staff. Some agents don't want to argue with pax in this situation (lack of confidence, lack of interest in the rules, sympathy with pax etc.) and will simply scan the boarding pass without giving it a second glance. If the agent does this to start with and some non-priority pax get wind of it, it's just a matter of time before the procession starts.

Once a larger group of people moves toward that direction, everyone further back will assume it's a free-for-all and go that way, too. At this point the agents will decide it's pointless to stop and just continue letting everyone through.