seriously, though -- why did this happen?
Well, dumbass didn't do it right. I'm pretty sure you're supposed to blow it out before taking the shot.
When he lobbed it into his mouth like that, it spilled on his lips, chin, and neck. He swallowed some, the rest spilled on his face, and he spit the rest out once he realized "oh shit, the fire isn't going out"...or it burned the inside of his mouth and so he spat it out. Thus, fireball created by the fire on his face and the spit alcohol as fuel.