The visitor tried to save it from drowning, after which reintroduction to the herd failed. Want to make it clear this wasn't a tourist just trying to openly grab a baby bison.
Still don't think this was appropriate (for the visitor to do) as the rule of these parks is to not interfere.
Often times, a baby animal who had contact with humans is rejected by the herd because it has traces of human scent on it. It smells like a predator. Birds display behavior like this too, which is why you should leave baby birds undisturbed if you find a nest.