The handlers violated the first rule of driving safety. Never, ever, ever have a horse hitched without having someone in the vehicle holding the lines. As soon as we hitch, we get a driver in the vehicle. That driver stays put until we stop to unhitch. A header can't hold a single horse much less a pair. Someone should have been sitting in that wagon while they waited for whatever presentation was going to made.
One comment about the wagon -- not very well made in my opinion. It was fishtailing like crazy at the trot on pavement. I wouldn't want to be driving a wagon that handled that poorly.
These people are very lucky that there weren't more injuries and property damage. From the video it looked like the wagon slammed into a least one parked car.