A short, but detailed story about the Pfizer explosion is here, from C&E News. While I was out on vacation, more details came out. I've been meaning to correct my earlier reports that the blast started with lithium t-butoxide.

It now appears that the cause was a drum of borane/THF, which makes even more sense. If something goes wrong with borane, you get plenty of hydrogen gas and overpressure. I haven't seen the stuff catch on fire personally, but I haven't been around a drum of it, either. THF burns just fine, though (I've seen plenty of that fire, I can tell you, and so have most other synthetic organic chemists.)

On long standing, I've been told, borane can start to react with the THF and open it up. I don't know if that's what was going on here, since that shouldn't lead to hydrogen production. Sounds like a breach of some seal is more likely. At any rate, they're fortunate that they got the fire put out quickly, since there was plenty more of the same stuff (and probably worse) stored nearby.