physiologically, what's going on when I vomit on the bike during/after a very hard effort? What's triggering that?

I've heard people say that when lactic acid concentrations get really high, your body thinks it has been poisoned and tries to get rid of the poison by vomiting.

When I google, I get lots of stuff about "be sure to hydrate during your workout!" I know that's not what's going on here.

Anyway, anyone know? I'm just kinda curious.