It was the Windsor Tri event today. I roughly know that road. Absolutely stupid behaviour but I can't help but think it wasn't intentional to pass up the inside. It doesn't excuse it at all but my bets are on a) they were drafting other riders and didn't realise until last moment / no one called out the horse(if it's draft legal? some of them are..) b) head down aero mode and they look up at last minute to see horse and riders passing them on the right so nowhere to go.
I always give horse/rider plenty of room and generally they're thankful. A fiesta driver pulled out on me earlier to immediately get stuck behind a horse 5 seconds later. She had time to wait and go round the horse at 2mph, with thanks from the rider and crew, but not to wait for me at the junction
I keep seeing this "head down" aero mode. If your head is down, you're not in aero, or at least not doing it correctly.