Why has my inlet manifold exploded on my alfa 147 02 reg?

I've just been told that the intake manifold on my Peugeot 307 2002 exploded.

Possible reasons given were: unleaded fuel going stale after long periods of disuse, wrong type of fuel used, or pressure of manifold cover. However, this was just speculation and I honestly have no idea. The last time I drove it it was over 100deg Fahrenheit outside (43 centigrade) and then when i tried to start it it backfired very loudly and that was that. A mystery!