Happened to me as well, I would also like to know

Either players, who subbed the game, lost their subscription/no longer subscribed OR your horse is too low level in Tier ranking and someone upgraded their Training Certificate, high enough to boot your horse.

For the latter, what this means is, if someone upgrades a Training Certificate from Regional to National? If your horse is a Regional rank? Well, unfortunately you cannot board with them no more and the game automatically kicks your horse(s). It's stupid and annoying to have Training Certificates tied into where people can board their horses at.

Highly doubt it is because you were inactive enough, couldn't afford the boarding center fees, or had horses at 0% health, all of which can be possible rules for boarding centers too.

Ok Thank you! and also for some reason all of the horses who were bumped were aged down? They are all foals again.

If the horses that were kicked, are suddenly foals again/deaged? 100% go make a bug report about it. 

We had a bug where horses would suddenly age up to 2 years of age, whilst frozen, so I wonder if that parlays into it, somehow

ok thank you