Here's the deal, according to Ross, whose Masters degree just so happened to focus on winter deer feeding in northern New Hampshire:
When people feed hay or grain, winter deer tend to pseudo-gorge themselves on the new food source. This is a shock to their digestive system, and, as the reader says, they don't have the living bacteria to digest it. People usually feed deer at a time when snow is deep and there aren't many other foods available. So the deer fill their stomach on the hay or grain alone and can get sick or die from it (rumenitis).