When a mob is pursuing a mob or player, the mob sometimes doesn't path find around any holes or lava in the way, and ends up falling into it.
This is a repeatable test. Put a villager on one side of a box, and a zombie on the other, with lava separating the middle in such a way, that it's the only path for the zombie to go to get to the villager. The zombie ends up going straight towards the villager and falls directly into the lava/hole.
Difficulty: Easy
Gamerule: doMobSpawning False
Yes, but they are Zombies, so gameplay wise it would make sense that they do that.