I have no idea why Pierre Garçon was fined for this play

The NFL handed out their weekly fines for Week 6 action, and Pierre Garçon has been fined $24,309, according to the beat writers. The fine came due to a helmet-to-helmet hit with Washington safety Montae Nicholson. Rob Lowder put together a GIF of the play, and I’d like you to give it a look below (or here if you don’t see it).

We see Garçon catch the ball and turn up field. Nicholson approaches to tackle him, at which point Garçon lowers his upper body. AFTER Garçon lowered his upper body, Nicholson lowers his own head and they make contact. Nicholson’s helmet flew off at the end of the play.

There was certainly helmet-to-helmet contact, but I don’t understand how Garçon can be forced to pay a fine when Nicholson was the one who lowered his helmet after the 49ers receiver. I could see how in the heat of the play a ref might have a problem making the correct call, but a fine comes after consideration by the league.

What am I missing?