An element of holes lining up (bomb threat, busy airport, back-track, fog etc) but ultimately a very impatient KLM captain - Veldhuyzen van Zanten - was the cause of this horrific accident.
You can tell from the CVR transcript that the KLM Co and Eng both had doubts about the runway being clear! The PanAm guys saw him bearing down on them about 10 sec before impact, but because they were taxiing so slowly (looking for their turn-off) they couldn't get out of the way in time. What a dreadful waste of life!