If your pawn tried to flee through the door, the aggroed muffalo could have followed them while the door was open.

Hostiles will try to follow their targets through door if they saw them enter. If the door is shut before they get to it, they'll try to destroy it, but obviously it's not the case here.

Edit : oh, just saw the log. The muffalo did try to break the door at some point. I'm assuming the pawn entered the room, the door was shut, the muffalo tried to break it, but then the pawn reopened the door again to get out and let the muffalo in.