The zombies take fall damage when they spawn after other zombies has taken damage (or almos it look like they took)
Steps to reproduce:
1. Take some zombie eggs.
2. Spawn some zombies in Medium or Hard difficulty.
3. Punch some zombies
4. Zombies will rain and take fall damage.
I think it's the same issue I have found in all difficulties (except 0):
Have burning zombies during day will spawn other zombies in the air