Why does Rafael Nadal pick his undies?

Answer 1

Much as this question represents low-class mentality, I would like to let you know that it is just a habit, like any habit any normal person may have, and Rafa is trying his best to get rid of it.

Answer 2

He is probably sweaty and uncomfortable after playing tennis.

Answer 3

In an interview with Uncle Toni, he says that its a nerves thing and that Nadal has been doing it for a while.