Depends what's in the feed. Eg. My horse is fussy with nutrient supps, and if for eg. I add powdered Mg to his feed, he'll usually eat it the first few times, then won't touch it, no matter how much other good stuff's in there. Even the pelleted nutritional supp I now feed, that they love straight, eat like lollies most of the time, they occasionally 'go off'. I put this down to them instinctively knowing what's good for them & if they've got adequate levels of certain nutrients already, they won't eat any more for a time.
But that this only happened once, perhaps the feed became contaminated? Is it possible rats got in & peed on it or such?
BTW, yes, horses can vomit. Speaking from personal - terrified - experience. When I witnessed my donkey throwing up once after worming, I freaked, having been taught that equids can't. Rang a vet, who said it was OK, just keep an eye on him, etc. Didn't believe him & rang 2 other good equine vets, only to get the same sort of advice. It seems that it is very uncommon for horses to vomit, because of the way their digestive tract is built, it is difficult, but not impossible.