Has anything new been introduced into the stables...new machinery..employee...different food....new horse. TThes are all things that can set an animal off. ALso I would check the area really well to assure no other animal has taken up residence in , under, on top or aroud the stalls. They next step would be bloodwork to make sure all their values are normal.
Good Luck!
Let me know if you have any other questions
Good Luck &
Best Fishes