He upped his finishing big time. It's a combination of Flick's influence, him getting more adapted as a main Barca figure, no injuries to stall his form, and things like that. It's not just one thing.
Main differences are the more direct, attacking football Flick likes to play, and him being more confident. His leadership, fitness, speed, stamina, basically all his athletic and mental attributes have always been impeccable and beyond reproach.
Moral of the story for fans here, never bet against leaders and hard working warriors.