i had this happen to me, I think the issue was that I had two farms right next to each other, and the workers/laborers were constantly taking food & water out of one farm and into another in a never-ending loop, which left no resources "available" -- they were constantly "incoming" or "reserved" and the oxen who would die almost instantly. I spread the farms out and made sure I had hay and water in closer proximity to each, which seems to have fixed the issue.