I first noticed this when I recently played Superflat Creative first time ever, when I saw a few Zombies drop from above (maybe other mobs as well, not sure about this), but that was in darkness and rain, and I presumed it might have to do with the Superflat world being close to rock bottom.

However, since then I've thrice witnessed single Zombies dropping out of the clear blue sky - literally - in Survival as well (Large Biomes world). The last instance appearently correlated with me finishing off a Zombie Pigman that had ventured into the Overworld; the earlier two might have happened in similar circumstances, at least I recall fighting mobs at the respective times.

What's going on here? Is Herobrine employing paratrooper mercs now?