Did not personally see the incident, but saw events before and around that per my post.
It was a group of 3~4 riders, someone said they came down from Bowen Ranch, though I saw them enter DCHS from PCT on the east side.
They left around 3:15pm towards PCT west side, though had an incident with a NiOC group dog.
Apparently the crossed the river at some point, then got ahead of NiOC, forcing them to go slowly behind them. (NiOC & Riders took more gradual slope path, I took the steep path up)
I encounter both group when less steep and steep path meet at around 3:45, of which the riders allowed the NiOC to pass them.
There was a rocky spot on the trail (where you can scramble on the rocks up, or take a narrow path on the left (right when towards bown ranch)) that had a boulder at the end (start of that section from Bowen Ranch) that you had to go around.
I talked to the NiOC, and we agreed that that spot in particular is too narrow for horse. Maybe that where they fell?
The link Katrina had said they fell around 4:30, of which I was already back by Bowen Ranch.
I mentioned seeing Fire Truck/Paramedic/Ambulance on my drive out and wonder that strange. Now I know why.