Once it gets down to a certain level, there is a probability of it blowing up. It could blow up right away, but that's unlikely. However, due to the slow decay, there is quite a few probability checks before the hit points get too low, so the chance of it blowing up while still having a decent amount of hit points can be fairly substantial. When the building is under attack though, the damage is done in a short period of time, and so there isn't much time for many self detonation checks to be performed. This means the building might not spontaneously blow up for quite some time. The direct attacks are usually more likely to destroy the building then the self detonation checks in that case.
I believe the probability of self detonation was fixed. So if you cripple a power plant, and then leave without killing it, it will probably have the same expected life span as one that was left to decay to just below the level where self detonation checks start occurring. (Provided it doesn't make it all the way down to 0 HP first).