Mobs that extend the Zombie class attack itself when being instigated by snowballs using command blocks

full thread
https://www.reddit.com/r/Minecraft/comments/5dr0a7/i_think_ive_got_a_weird_bug_that_i_want_to_submit/

Summary (thanks to [Mod] Skylinerw)
It's because the zombie is attacking itself, funnily enough. Zombies all attack its offending target without losing focus, so by causing one zombie to attack another, the attacker starts attacking itself. You can confirm that in-game by modifying the attackDamage attribute of the zombie hitting itself.