One of the reasonings could be inactive for more than a day, or X amount of days, horses were frozen for more than a day, or X amount of days or there was bad payments for more than a day, or X amount of days. The finality rule could be you log in but do not care for your horses and they drop 0% health, there are some boarding centers out there with 1 day rules/0% health rules.

One of the other reasonings could be, the owner used to be an upgraded member but is suddenly basic/membership ran out etc and once they downgrade to basic, they lose access to their boarding centers.

As I mentioned in another help post, you could try finding boarding centers that have more than 1 day rules and board there instead or follow the rules the owner sets up.