I believe the way the language is on this....
A high seniority cover driver can bump a bid driver off his route, only if there are no unassigned routes available.
IOW, the high seniority driver would have to 1st bump a lower seniority driver off either an unassigned route or a vacation route before being able to bump a bid guy.
I have seen it happen in a neighboring center. A rural route was converted to a remote route. No one wanted it, so it was forced on the lowest driver in the building. 4 months later, it was moved back to the building, but not rebid. So the lowest seniority driver in the center has that route.
In the winter, when no one is on vacation and volume is low, he is bumped off his route for 2-3 months until vacation start or volume picks up. At that building, it will probably be a few years until that driver is not bumped off the route in Feb-April.
So, it can be done, but it is generally rare.