The NBA has sent a memo that jumping on the back of a player during free throw attempts will be called as a Flagrant Foul.

"We have recently seen instances during games in which a player, in order to commit a deliberate foul, jumps on an opponent’s back during a free throw attempt.  This is a potentially dangerous play against a player in a vulnerable position.

"Please be advised that the referees have been instructed to evaluate such plays under all applicable playing rules, including the rules relating to Flagrant Fouls.  Players remain free to commit deliberate fouls during free throw attempts, but such fouls will be assessed as Flagrant if they meet the applicable criteria."