I can't find it online but local papers are carrying a headline which states that investigators have confirmed (more likely someone has suggested) that the accident may have been caused by a balloon which was photographed in the vicinity of the plane just prior to the incident.
It looks to be a large hand held novelty balloon which has escaped from its owner, at a glance.
Interesting if this could indeed have caused a problem with the prop - there were quite a few floating about during the display.