Fox was unseated after his mount, Luxford, veered sharply to the right.
Television replays showed the filly had been distracted by a stray golf ball on the course, which appeared to be thrown up by eventual winner Big Lachie.
The ball ricocheted off the helmet of another rider and across the sightline of Fox's mount.
Sandown Park in Surrey has three nine-hole golf courses close to the racetrack, but clerk of the course Andrew Cooper said they were not open on racedays.
"I am satisfied that we take all reasonable precautions to make sure that the course is as clear as we can make it but we accept that there will be some balls buried in the grass," said Cooper.
"We have a 14-strong team that walk the course, and other parts of the track, looking for golf balls.
"Even where there is netting next to the driving range we have found them. It is not a unique occurrence and we probably have this happen once every three or four years."
The afternoon ended on a more positive note for Fox as he partnered First Experience to victory.
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Jockey Kieren Fox escaped unhurt when he fell from his horse in a bizarre incident involving a golf ball during a race at Sandown.