Picking at the seams: It's in the rules...

Law 35: Hit Wicket. One of the more common among irregular means of dismissal, a batsman is deemed to be out if "while the ball is in play, he breaks his wicket with any part of his equipment or person while preparing to receive the ball, playing a stroke, taking evasive action or setting off for a run immediately after playing or playing at the ball".

At one time, a batsman had to be playing a stroke to be given out. But given the difficulty in deciding when a stroke begins and ends, it was extended so that a batsman slipping over in setting off for a run can be out if he kicks down his wicket in the process.

But even a clean hit and sure feet are no guarantee against this form of dismissal. In 1865 an Oxford University batsman was given out "hit wicket" when part of his bat broke off, hit him on the head and fell on to the stumps.