When a horde spawns, they all appear at once, smashing furniture, and making such a racquet that the player character is deafened for a very long time. (permanently? IDK I ded.)
not permanently, few hours.
Regardless, shouldn’t happen.
edit: looks like I misinterpreted what was going on. The mall was collapsing due to damage sustained. Might not have been the arrival of a horde specifically, though I will add that the mall seems especially susceptible to collapse.