You need to ask why the chain came off not why hill climbs might be stressing the bike - the chain is obviously what screwed the stay, not vice-versa. Possibly the hill climb caused the rear wheel to slip and the chain was then running loose and waiting to climb off, but I wouldn't see this as a failure caused by "over stressing the bike"... mega slack chain fail?
You need to ask why the chain came off not why hill climbs might be stressing the bike - the chain is obviously what screwed the stay, not vice-versa. Possibly the hill climb caused the rear wheel to slip and the chain was then running loose and waiting to climb off, but I wouldn't see this as a failure caused by "over stressing the bike"... mega slack chain fail?