That's not a flop. Manu just tried to jump forward and hoped to draw contact and a foul. Tony Allen did a nice job in staying straight up and backed away from any contact. Manu basically lost balance and fell when there was no contact.
I tell ya... I really hope they crack down on the NBA flopping. Manu (and players like him) should be sidelined for a game if tape shows that it was a really blatant and terrible flop.