In mine craft, if a zombie is behind a door, and if you attack another npc near the zombie behind the door, it will walk up to the door, and is able to attack you from behind the door.

While playing the game, i was mob grinding for xp, and i went to my mines which is about 15-25 blocks away from the mob spawner and there was a zombie at my mine entrance door. I was going to open it and kill it but when i got to the door it hit from the door. I even went to my creative world and tested it out on all the doors including doubled and single doors.

Steps for this to happen
1: Have a zombie behind a closed door.
2: attack or kill another zombie out from the door.
3: Go to the door.
4: Bam! get attacked by the zombie.