Well, this is scary. Several videos have surfaced of a chairlift rollback in Gudauri, Georgia that left several people injured.
The video show skiers and snowboarders literally being launched from chairs as it speeds backwards.
Here's how one member in the forums described how rollbacks can happen:
elliot.scribnerThey are more common on old school fixed line lifts. Basically what happens is the driveshaft or gearbox on the lift fails, causing all of the weight of the people on the upward slope to quickly rush down the hill because gravity is pulling all of their added mass down (since the other side has no mass on the downward facing lifts). Lifts have emergency brakes, but those can fail too, and they take a some time to kick in. Highspeed quads have much better technology behind them, nearly eliminating rollback risk.
More info in this thread: