My QH mare did this quite a bit last winter, and I was totally perplexed. She was (and still is) on 6 acres of pasture, and was splitting a bale of hay a day with 2 other horses, so always had something to eat. She has always had access to a mineral block and plenty of water. She would only eat the manure of the dominant mare in the pasture, and would eat up almost every pile the mare made (but ignored her own manure and that of the weanling filly). I put her on Accel, which contains minerals, vitamins, and probiotics, and she stopped within a few days. She has only done it once or twice since then, and each time when I forget to give her Accel. I really think my mare had some sort of intestinal pain/imbalance (but not to the level of colic), and the Accel took care of it. Just a thought.