I'm drifting and I don't know why

As a longtime Mario Kart Wii player, I always used Automatic drift mode so I could concentrate on the higher-level details of what I was doing rather than how I was doing it. (That's how I rationalised it, anyway.) Mario Kart 8 doesn't have an Automatic mode, forcing me to take a (ahem) crash-course in drifting. Anyway, while I was figuring this out, my bike would occasionally drift, sometimes long enough to Mini-Turbo, all by itself. I'm not kidding - I'd turn the GamePad left (albeit hard left) and the bike would inexplicably drift. (My younger brother, who had drifting worked out before I did, didn't believe me until it happened to him on the same bike.)

So far I've reproduced this behaviour on three bikes (two drifting inward, one outward), two karts, and a quad, which leads me to suspect it's universal. So here I was thinking ZR was required to drift, and then the thing occasionally does it itself - anyone know what's going on? I've read the entire electronic manual (yeah, I'm one of those people), but still cannot find any mention of this.