Sudden Collapse

My 7 yr.old horse collapsed just as he was being injected to reduce swelling on a hind leg.His legs thrashed around but he couldn't lift his head.The fall cut open his head just above the eye causing some flesh to spill out and dangle from his brow.
   
The doctor doesn't know why he collapsed as the meds didn't have a chance to go in yet.The wound is now horrendously swollen and pus is draining from the eye.He has never collapsed before and has always been a good patient when receiving his shots,so i know he wasn't under stress.The only complaint was the mild swelling in the hind leg.
   
Was this in any form due to human error as the Dr. was very junior and had only been a practising large animal vet for 3 years and that was mostly on cattle.Please Help.