They explode because the crows in the area are picking out their liver's to eat them(Yea, they have figured out how to do that), and the toad's defense mechansim to this is to puff up, naturally. But because their liver is missing and their blood vessels are all screwed up, they just keep on expanding until they explode!!!!!
EDIT: No chemicals or other negative-effecting things have been found in the water or habitat.