Being so close to the boarder, most of the unsold, broken down, unsound, old, or too-hyped OTTBS get sold at auction to meat buyers for $10-30 dollars a head. Sometimes the trainer will be gracious enough to just have the animal euthanized, but it's generally the meat truck that takes them away.
That's why we have so many rescues such as The Gold Horse Ranch, and my personal favorite- L.O.P.E. They take in and rehabilitate horses with no chance otherwise. Really a fantastic place, and if you ever get the chance, you ought to read their book. It's a great piece of work.