@Riamus ah ok my reference still was that scene where they were obviously after that guy presenting himself right on the other side of the fence.
So your example would be more like a weird spawning thing? I mean, if you are far away there are no zombies, they start to spawn in the area you move into.
And in your case they then seem to spawn in places they get stuck and get into "destroy area" mode or something.
I have experienced it myself that the zombies seem to be a bit more likely to get stuck somewhere, preferably in rooms, even in open doors, and just keep hitting the surrounding blocks instead of moving the obvious way.
Seems that pathing needs a little polish in A21.