I'm not a skydiver. A. A hook turn is a quick canopy turn performed near the ground. Turning a parachute makes it bank and banking increases its rate of descent. The harder the turn, the greater the bank and the greater the descent rate. The jumper swings out from under the parachute because the parachute is turning and diving while the jumper tends to go in a straight line.
Some jumpers like to perform hook turns because they're fun, like driving a car at the edge.
The ground is very hard, and many orthopedic surgeons are putting their kids through college on the money they made off of botched hook turns. But other skydivers perform hook turns somewhat inadvertently. Many jumpers have never received training on how to make the best of a bad landing situation. It's also much better to land under control on pavement than out of control on grass. Hook-turn injuries are almost always preventable, and there's nothing mysterious about them.