Seems like it was the result of a paddock accident.
Thorougbred Daily News
from
Alicia Wincze on horse racing:
Michael Jester, owner and syndicate manager of Penn Ridge - which had stood Real Quiet since 2006 - said the son of Quiet American had been turned out briefly in his paddock while he stall was being cleaned when farm employees noticed he was laying down and not moving.
Jester said an autopsy determined Real Quiet had fractured his C5 through C9 vertebrae.
“He was always so calm, we’ve never had a problem with him, he was such a class act,” Jester said. “One of the farm employees said he was laying down in his paddock, which he never does, and we ran up to him and he never moved from the time we found him.
“He must have reared up and fallen on his left shoulder and driven it with enough force to fracture those five vertebrae,” Jester continued. “There was not a mark, not a slide, nothing. Just a completely freak accident. We’re devastated.”