My mare did that when I first got her so I just moved her buckets to another spot (she's not in that much anyway). When we moved to another state she did it again where I first hung them so I moved them again and she rarely does it now. She just likes to poop in 1 spot and that happened to be there the buckets were.

I'd try moving the buckets. It maybe inconvenient to fill them but it beats cleaning the s*it out of them every day!