I'm drifting and I don't know why

I just tested this as well (with a standard bike; you appear to have tested with other karts enough) by doing donuts in Dry Dry Desert. I do indeed see the same automatic boosting behavior on sharp turns.

This appears to be a subtle feature of the game that isn't obvious or well documented.

I can't say "why" this exists without consulting the developers, but if I had to guess I would say the automatic turn boost is there to assist with turning in situations where you weren't able to pull off a drift - drifting always goes to the outside (even with sports bikes, during the early part of the drift), which means in some situations you might not want to risk drifting. In those cases, this turn boost could kick in.

It's pretty hard to do on purpose, but when it happens it can only help - as far as I can tell, other than giving you a speed boost, this automatic turn boost does not affect how you turn.

Mario Kart 8 merged the 'Manual' and 'Automatic' drifting modes from Mario Kart Wii into one. You can use the ZR trigger to perform manual drifts, but if you don't, automatic drifting will be engaged. All vehicle types support both kinds of drifting.