Mobs not realizing they killed their attacker and trying to attack empty space

Iron golems will occasionally attack creepers, causing them to explode. The golem will then continue to try and attack the space where the creeper was. Unloading the chunk will fix their behavior. The golem is probably not 'receiving credit' for the kill when the creeper explodes, so they continue to attack.

This behavior applies to all mobs with the "new" AI that are capable of attacking (skeletons, zombies, wolves, witches). Basically in an infight if one mob dies the "killer" will still try to attack the empty spot where the mob died.