Usually a combination of terrible things all happening at once.

A foreign flight to a smaller airport, very late, but union regulations have the customs agents off the clock at that hour and the place is not a big international with 24 hour customs shifts..so by law nobody can really set foot in the country, and the airport itself doesn't have a fully secure airside..and the plane lost it's gate by being late and the place has no airstairs..etc etc etc.