Do we know for sure it was the crossing that made him go around?
There could have been a number of reasons for it, eventhough the crossing seems like the obvious one.
The landing is the responsibility of the pilot, and even if tower calls for a go-around, the pilot is in his right to elect to land.
In theory, and in practice, the pilot can elect to land on an occupied runway if he deems it safe, or safer than executing a missed approach.
I've seen it done in practice with a "big jet", where a vehicle has moved beyond the stopline on a taxiway in the far end of the runway, but has halted short of the runway itself, and a go-around was called by tower to a "big jet" on very short final (including information on the vehicle).
Ofcourse, something moving onto a runway half-way down, would initiate a go around by most big jets, but smaller aircrafts may still elect to land.
Now, this time I keept my own "small aircraft" piloting experience out of the picture, and referred to a real life experience as a controller with "big jets".