Fair catch?

I was watching the Denver game. Chargers punt. Denver calls for a fair catch. One of Denvers players is pushed into the player calling for the fair catch. He fumbles obviously. Chargers recover the ball.

How is this not a Charger penalty? Why don’t more teams push players into the punt returner?

 

It’s legal which is what the commentators said. The punt returner is supposed to scream and let his teammates know to clear out.

 

Thanks!