Noticed that zombies were spawned in the houses (coming from nowhere when you're standing pretty much in that spot) and they were glitching through the ground and walls.
Also, when following you (I was running like hell) they'd glitch through the walls and fences.
Maybe an idea to make sure that zombies can't "open" doors (so you're a bit safer inside) although maybe break down? If they cannot jump, make them move around the fence rather than let them glitch through.
Cheers!
Alyngwen_C