Have seen a couple of cases of Red Maple toxicosis in horses through the years... in my experience, it's almost always been when someone(or their neighbors) has been 'pruning' the trees in their yard and pitches the limbs over the fence. It's a bad deal.
See similar disastrous results with folks pitching Japanese yew trimmings over the fence to give the cows a 'treat'... end result... a bunch of dead cows.