one explanation as to why that beautiful horse had to be "put down".

It's a shame that the level of competition between humans drives us to such lengths. When we compete to a level it's obsessive and injurious with our fellow humans, it's one thing, but to include animals in our pursuit to the extent that it injures or kills them is beyond my scope of comprehension.
Pity, such a beautiful animal.

Well said, crobinett. Thank you.