I would expect either a foul (Sexton running into Curry), or him being out of position to defend the shot, and having to essentially stretch backwards to get a hand up since his momentum is carrying him forward.
Basically, he should have overpursued (for lack of a better word), and had to stop and kind of dive back to get a hand up.
Basically, this:
Defender gets beat and has to sprint to catch up. Harden hits him with a stepback, and the defender runs past him. He has to plant his feet to try and defend the shot, because his momentum is carrying him forward.
Then, because he had so much forward momentum, the most he could do is stretch back and throw a hand in the air, even though he really isn't close enough to put up a good contest.