Theories include asteroids and hunting. There's no concrete proof on the first. Ice age hunters needed extraordinary motivation to kill mighty beasts with mere pointed sticks. TheRaven's keen study of modern human behavior has led to a breakthrough theory of the mammoth's demise. By impersonating a crow and studying behavioral differences between human males and females, TheRaven was able to extrapolate backwards to root cause.
The mammoth were doomed the minute humans invented language. Important to this theory is realization that women must have invented language. Men had pointing and grunting and, given modern male behavior during football season, it's quite clear that, without women, men would still be pointing and grunting. Before women had their linguistic eureka, the mighty male hunters usually sat around the fire, worrying about their place in history. With grunts and gestures.
"Oy! Are we mighty hunters or are we going to spend eternity killing rabbits?"
We should kill mammoths!"
(Ice age hunters, in fact, mostly hunted arctic hares and on that score, weren't so mighty).
Other hunters said...
"Are you bleedin' mad?"
(Oddly, they gestured and grunted with British accents)
"All we have are these poxy sticks!"
During one such exchange, at the other fire, women invented language. When they realized they could communicate with more than grunts and gestures (which was always less suitable for women) they cast about for topics...
"Let's talk about our feelings!"
The women all looked at each other, instantly realizing the power of language to leverage their feelings into a prehistoric form of remote control.
"No, how about....relationships!".
Shock at the existence of relationships quickly gave way to realization of the ultimate man-control. Smiles and nods all around were slightly tinged with sadness as the women realized that, at least in a fashion context, being able to talk about shoes was a few years in the future.
The men weren't talking yet, but they weren't deaf. Not knowing exactly what feelings or relationships were (many still don't), but fearing the worst, they frantically gestured and grunted to find a way out of their new predicament ....
"Er...hunting mammoth takes a lot more time than hunting rabbits".
Questioning looks around the fire.
"Yeah, that's means we won't be around for (fearful and confused looks all around) feelings and relationships!" (terror stricken looks on all their faces).
They stared at their pointed sticks with desperate religious fervor.
"You know, if we all stab the mammoth at the same time....it might work!".
Much relieved at dodging the terror of feelings and relationships, not to mention learning how to speak, the hunters truly became mighty and laid waste to the mammoth.
With pointed sticks.