Lean angle has a bit to do with it too....the further your over, the easier it is to spin up the rear, especially in the slow to med speed turns, the rider tries to get outta the turn quicker, prolly grabs too much too soon, and when the rear steps out to far the throttle usually, but not always gets shut off, which is a mistake..The best thing to do is either keep the gas on, or gently roll it on further. When you see racers spinin up the rear, they are usually more upright, also they have THE feel for keepping the gas where it should be, so they don't wind up on their head...Don't ask me how I know this..
If you happen to be leaned over, and spin the rear, just stay with it, your chances are better than to get off the gas.
I would take exception to Grashopper's example that if the rear slides, your prolly gonna highside. This really depends a few things, i.e. the speed, lean angle, the turn itself, and the rider.
Many times, I've seen a bike get sideways, and lowside, I also have experienced that too. Prolly the easiest turn to get into a bad situation is a downhill turn, where alot of the weight gets sifted to the front of the bike, hence the rear is unweigthed, and the rear spins up fairly easy..
Oh, one other thing...in the days of Grandprix raceing when they used 2 stroke 500's, you'll notice MANY more highsides, as opposed to nowdays, cuzz the power delivery on the 4 strokes is much nicer, i.e. a better power curve..