There could be variety of reasons, some of which have already been mentioned. Another reason can quite possibly be that the door on the plane decided to stop working. Plus as many of us know depending on the airport, gate, etc., you are stuck with only using a specific door on a plane and cant just use any door. I had something like this happen once a while back on a DL MD11. We were stuck of almost 2 hours on the plane before they could get the door open. Stairs werent an option as there were none around that they could get "high" enough to safely reach the door of the plane.