There are two schools of thought - one is that it was an enormous biochemical reaction from some substance in the insect's bite. The other is that the insect planted eggs in her, and it's the larva coming out that made her pop, which is why they needed to isolate her in the biohazard tent.

I thought they just stuck her in there so, y'know, the huge crowded room wouldn't get showered in blood.

But why wasn't there a whole slew of gunshots or an effort to really contain it? It was over so quickly, and the soldiers didn't step in to eliminate anything that might have come out of her... It just sort of happened.

I guess the biochemical thing sort of makes sense though. Considering how the creatures rampaged around, they may attempt to do whatever they can to quickly kill their prey and even eat carrion for those that are bitten and die. Or, it was just a bad reaction to space herpes or somethng.

So, any thoughts on what exactly the Cloverfield monster was? Crash landed alien creature? Ancient monster from the pre-cambian period? Mutated radioactive brine shrimp?

This explanation doesn't make an iota of sense, but I heard that some Japanese corporation was messing around with genetics, and launched some kind of rocket (which is what you see crashing into the ocean in the last scene) that contained some kind of experiment that accidentally created the Cloverfield monster.

Hence the "iota of sense" disclaimer

EDIT: Oh, I didn't the "at the end" part of your sentence.

The rocket hits at the end of the film, but as pointed out below, the video that was recorded over happened much earlier in the year (several months, if I recall).

That part of the movie stuck with me the most, and really creeped me the hell out. Combination of eyes bleeding and the quick isolation by the military maybe, but, man that was freaky.

I'm putting my money on chemical reaction. And/or dark magic from an antient one.

According to an interview I saw with the creature designer (I think that it's a DVD extra), his brief was to create a "baby" amphibious monster that had been been seperated from it's parents and was panicking, which leads to the conclusion that it was something from deep down that had been scared onto land by the earthquake.

It still makes very little sense (if it was from so deep that we had never seen it before, surely it's foray to the surface would have ended with a sudden "pop-splat"moment?) but I really liked the whole "let's not go into it" vibe from the film. After all, that's really not what it's about.