The incursion was by a fire engine which was hurrying to handle yet another incident. The weather was foggy, it was raining, and the incoming plane was already low, so it was pretty hard to tell it apart from many other lights shining from the fog in the distance. It's not easy to assess the speed of motion when a fuzzy ball of light is advancing right towards you.

The pilot was given the clearance to land before the fire engine was dispatched. Apparently there was not even enough time for the crew to max out the thrust and try to lift off the strip even if they managed to notice the lights of the incoming fire engine.