User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-74.193.207.108-20120913212250/@comment-4212769-20120917205122

I think it isn't only heat (if at all) that causes the explosions. It's more likely pressure. Without pressure it doesn't seem to react and come to an explosion. It isn't completely fluid but very viscous. Through this viscosity it absorbs the pressure it gets through the punch/hit on the ground or body of something. That's why it explodes in the Tundra and the Deserted Island as well.

Like I said before the spit is the explosive substance but it needs the slime to actually explode. The slime works as some kind of catalyst in this.

About why it doesn't explode when Agnaktor hit it with his laser it's probably that it has some kind of enzyme or bacteria on the skin and/or claws that restrains its reactions or completely prevents the substances of exploding. But when Bracchidios hits the ground with his claws or horn some slime clings to the ground/body of something and the enzymes are gone. So it can finally explode.