I've been in the trade for over 20 years, and i can tell you that they don't just blow sometimes... the only time they "just blow" is if there was some sort of damage inflicted upon them, whether at install(40 years is a long time for a damaged sprinkler to hold lol) or there was a flying object that struck one in the shop. There's another possible explanation too, there could have been enough heat generated from someone torching parts or even playing a joke with a bag of flammable gas that got lit... I've seen that one before. Nobody wants to admit what actually happened is what i find in most cases, then the old "they just blow sometimes" is the go to response, which is not true. The sprinklers are air tested before they leave. A malfunction happens about one in 10 million and doesn't take 40 years to find......