I think it’s based on the speed of the drinking. If beer is your usual drink, you don’t sip, you quaff. So, when you get nicely toasted on beer and switch to whiskey drinks, you might keep quaffing. You’re going to get really blasted. It’s even worse if you start slugging down shots; that’s high-speed drinking.
So, if you’re accustomed to going home after a six-pack of beer, and one night you have four beers and eight shots of Cuervo Gold, naturally you’ll wake up feeling like you fell out of a tall tree and hit your head on every branch on the way down.