There is a somewhat grainy picture here from the LA Times. There is an eyewitness comment where he refers to a hole: "...I was there and saw the big hole they fell through". So it sounds like the bridge separated from the aircraft or the floor fell away.
@Martin, interesting the last sentence in that piece read "The plane was damaged and towed to a hangar." I presume the bridge must have damaged the door or fuselage as it fell away.
The pilot was exiting the aircraft at the same time as a passenger? ?| Crikey he must have been in a hurry. I've never seen the crew beat passengers off before.