Exploding iPhone?

A couple months ago I put my old iPhone 3GS away in the drawer where old electronics go to die in our house... I'd used it for several years, my daugher had used it for a year... it had served well and we'd moved on.

The drawer is inside our house and nowhere near heat nor water/moisture.

Today I opened the drawer to find something and instead discovered this -- see attached photos. The phone split apart at the seams, apparently from the inside out. The glass is intact. The back is intact except that what used to be a small crack by the earphone jack is now a bit bigger. The whole thing is just blown out.

I'm assuming maybe it's somehow battery related?????

Anybody seen an iPhone do this before?

I'm glad that my house didn't burn down, if in fact whatever blew the phone apart involved heat.....