Zombie Villagers Breaking Iron Doors

While keeping Zombie Villagers in a cure chamber on hard difficulty, they would begin to kick down the door and the same animation would play as it it were a wooden door being broken down( with iron door particles) and normally stop, but however, about 40% of the time they would successfully destroy the door. On one occasion, there were two in the same chamber, and they managed to break down the door. This also happened underground around level 30 (just below the bottom of the ocean floor) for what its worth.