To Reproduce:
/gamerule maxEntityCramming 1
tp @e[type=!player] X Y Z (Looping command block)

All living entities die other than bats, no matter how many.

Possible Reason:
Entities have a hitbox and you can hit them, but they do not "contact" other entities. If the max cramming is set higher and other mobs are in the XYZ you set, you will be pushed back when you walk into them. When it's just the bats, you can walk through them as if they weren't even there.