Here's a video of someone closing a 777 door. It's relevant that the person is a woman, as in this accident, and therefore has a fairly short reach and slight build (compared to most men).
This is fairly clearly a training article, so the door doesn't close as well as the real door, but it shows the technique.
Notice that you have to reach quite far out to grab the door operating handle to bring the door in. There's a handle on the door frame and the correct way to do it is as shown: grab the frame handle, brace your feet, lean out and grab the door handle and then lean back in, pushing on the frame handle and pulling on the door handle to get the door to move.
Now imagine if someone didn't brace and lean back, or the floor was slippery, or they leaned out with both hands to grab the door handle. They would be close to falling out and poorly balanced - especially if they used both hands on the door and just tried to pull it in.
It's a 5m drop to the concrete from the door sill if there is a gap to the stair or airbridge, or those have been removed, or this is a door opened for ventilation or service (galley, etc) rather than next to a stair or bridge. Definitely serious injury territory, death if you're unlucky (landing on your head).