Or, owner has lost confidence through accident / injury that wasn’t the horse’s fault.
For example: Fall over a jump (rider), fall off another horse (now giving up riding), injury that no longer permits owner to ride / be comfortable.
Or owner has lost job / downsizing / divorce / lost interest in riding / changing discipline / lost agistment / moving…
Or the most common of all: rider has outgrown in size / ability.
Plenty of reasons that a horse ends up for sale through no fault of his own.