This is a fairly well known aspect of the game that has been dubbed by the players as “nav meshing”. This happens in a lot of places, there are many more places in Righteous Stand, as well as many places in all other maps. As far as I understand it, the game has a navigational mesh that defines where valid locations for enemies are and is used to allow enemies to path from location to location. Nav meshing happens to an enemy when they are pushed into a spot that is not a part of the nav mesh, meaning they are unable to path find out of that location and are effectively stuck. The game sees this stuck enemy and automatically kills them so they do not take up computing resources or stall out hordes/events.