This has to do with the pathing system of the tank. Think of it this way: the tank has a series of waypoints it travels through to get where it needs to go. Only when the tank reaches a waypoint does it determine if its “broken.” This is why the tank can run out of HP while crossing the bridge but still make it to the other side before breaking down. The damage occurs before the tank has time to reevaluate if its broken or not. This is done so that the tank doesn’t break down on the bridge, and cause all sorts of problems if the axis manage to blow the bridge a broken tank is sitting on.
Anyway, if the tank runs out of HP between waypoints and an engie tries to fix it before it “breaks,” something goes awry and the tank gets confused. So it just sits there with no HP and not breaking so anyone can fix it. Eventually it figures out what’s going on and finally breaks down. To avoid this in the future, never try to use the pliers on the tank until after you get notified that it has broken down.
RK