Fascinating, Sherman may be right. Also, a quick google search finds that some dessicant explodes when soaked in water (rather than just swells) - though it's hard to imagine enough force created by this to cause the damage here.
I also find that most dessicant is de-activated at temperatures exceeding about 350F. Internal fixture temperatures can easily exceed that.
If a chemical reaction was the cause, in addition to Sherman's thought about Mg from the Al, there is significant outgassing from some types of grease used in sockets - this can be seen as fog deposited on the inside of the lens.
And, Sherman mentioned reactions with Xenon - that, of course, is a gas used in some halogen lamps - if the lamp capsule cracked - boom!