It could be a number of different things. Make sure you have them on a good deworming program. Being back east I doubt that you have to worry about sand and feeding Psyillium. THat is always something we have to worry about here in the desert. Her food could be to rich for her system and that can give her the loose stools. That would be an easy place to start. Some horses also just drink more water than most and this can cause their stools to be loose also. You could try a probiotic paste or powder also but first check with your vet.

Probably the best thing to do is give your vet a call (since phone calls are free) and talk with him/her and see what the vet thinks you should do. Make sure to explain everything to you vet. Her feed, stabling, turnouts, other animals she is with or around. All of this will help the vet try and figure out what is wrong.

Hope this helps good luck.

Just a thought, my older horse tends to gets loose stools when the wether changes.