Can't say what's causing it, but I resolved watery manure completely in my horse by doing a couple things.
First I took him off all long stem fiber, and had him on just pellets. The watery manure went away right away. I was able to introduce chopped hay a few days later. I cut some hay by hand into gradually longer and longer stems over a month or more until he was back on half pellets and half hay without any issues. Waited a while on that combo for another month or more and then switched the last feeding of pellets over to hay. Haven't had any problems since.
I also give him alfalfa pellets as a carrier for Daily Gold, a clay supplement, and I was giving him psyllium daily as well, but recently stopped the psyllium since it's been several months and I'm sure if there was some sand hiding in there it's gone by now.
He had nothing showing up for sand tests, had been wormed recently, and got the ok by the equine dentist and vet while he was having the manure issues. He had these issues for several years before I got him and started doing test after test to stop it in the time I've owned him.
I used baby wipes to clean off his bottom, or a warm rag if you can manage that. Then I applied cortisone creme to the raw areas down his legs and would massage it in. I would also apply coconut oil to help the manure slip off the hair, which sometimes worked and sometimes didn't. Just to give a break in the steroid use.
Using mucilaginous herbs such as licorice, slippery elm, and mallow will create a barrier in the stomach lining. If it's from irritation to the GI tract those should help. I used the daily gold because it will create a similar type of slip, and also being basic, will help buffer any excess acids and move built up toxins through. I am not sure of the acidic or basic properties of mucilaginous herbs, however they should help clear up anything that's gotten trapped in there, similar to psyllium, which is also a mucilage.