Pietersen was out when his helmet fell on his wicket
This dismissal happens when the batter knocks their stumps over while playing a shot or avoiding a delivery.
This can be with the bat or with the body.
It often happens when a batsman steps back too far playing a difficult delivery and accidentally treads on the wicket.
Kevin Pietersen was once out hit wicket against the West Indies in 2007, when the chinstrap of his helmet broke and the helmet hit the wicket after it fell off.
The bowler gets the credit for the wicket.