I can understand how it happened, but not why it became such a problem to undo.

Push the flap down, then you catch your finger(s) between the flap and the rim. The harder you pull, the worse it gets jammed.

What I don't understand is why the pax couldn't push the flap back down, hold it down and open, and pull his finger out.

If the pax pulled too hard, the flap may have actually started to swing up beyond the closed position. That could result in a jam that would become very very painful to reverse.