Player made iron golems randomly attack/kill passive mobs like cows and pigs and sometimes villagers.
How to reproduce:
1. Build iron golem
2. Leash Iron golem to fence or put in enclosure
3. Spawn passive mob with iron golem (ex. Pig)
4. If the pig walks into the iron golem the iron golem hits the pig

This happened to 1-2 villagers, a cow, a pig, two mushrooms, during day, easy difficulty.