Some horses end up in rescues because their owners are unable to care for then, usually an elderly owner who either dies or has to go into a care program. They or their family end up surrendering them because they can't be sold fast enough.
And it is why I am down to two horses, having recently laid my 27 & 29 year olds to rest.
My other two are 21 & 22. The 21 yr old has serious and costly health issues. I am 69 and "Sheriff & The Good Lord Willing", I will keep my health to see both of them to their end times.
There are a handful of ethical people in my county, including the vet facility, who know both these horses get PTS'd and laid to rest on this farm, if something unexpected were to happen to me and DH is no longer "available".
2. Divorces can also bring about abandoned horses. I was in my early 30's when I rescued a registered five gaited Saddlebred out of a boarding barn.
The BO was barely feeding the horse, the couple were in the middle of a nasty divorce and nobody had paid board on the horse for a few months. I managed to get him and his papers for what was owed in board -- back then, that wasn't much.
3. Job loss is another big one.
Anything can happen to make a caring home suddenly go south.
When I read about all the horses fallen on hard times due to their owners falling on hard times, I thank my Lucky Stars, I was able to hang onto not one, but two sets of Keeper Horses in my lifetime. And there were times as a single mom I'm not sure how I managed.