He could have eaten something he wasn't supposed to and his internal organs may have begun failing from it.
For it to have gotten him so quite I would guess anywhere from posion, to glass shards.
Recently, there has been a rash amount of animals dying not far from here. Someone was out throwing posioned meat around randomly, but also was throwing poisoned veggis and such!!!
Dogs, cats, chickens, deer and birds were showing up dead within a 5 mile radius of a town near me.
They have found some of the meats thrown out... but they were on the side of the road and in wooded areas.... someone was out driving around with it!!! I hope they catch them.