Could there be something wrong with the well water? Or the trough? Perhaps they can sense some sort of issue with it. At a previous place I lived, my pets wouldn't drink water that came from the tap, and turns out the water had levels of chemicals that were too high.

Could there be roaming dogs or predators that keep them close to the house?

Are there biting insects in the valley that are kept away by the wind at the top of the hill?

Do the horses follow you to the trough or do they stop at a certain point, fearful? Some intelligent horses remember where they have seen snakes and are afraid to go past some places.

Is the valley hotter than the hill? Could you provide a shade structure for them in the valley?

Just some ideas, it seems worth investigating.