I was intrigued to see film of the incident, which does put a different perspective on the incident.
The way the media appeared to portray the incident, it was made to read as though the horse was punched in the face, similarly to the Blazing Saddles clip, rather than hitting its flank.
I am nothing of a horse expert, so I have no idea if a blow like that would hurt or upset the horse, but it did appear as though the horse was already out of control.
The actions of the coach were clearly not helpful, and equally clearly motivated by frustration, rather than actively assisting in trying to get the horse under control.