The answer to this question is found in chapter eleven of the text.
Mr. Lewis, the owner of the barbershop next to Carter’s Grocery, is having a conversation with Maverick about why Mr. Lewis identified King to reporters when questioned about an attack on some policemen by members of the King Lords. Maverick doesn’t quite understand why Mr. Lewis would put himself at risk by doing so.
However, some nearby police officers see the two men talking and assume that Mr. Lewis is being accosted. They intervene, asking to see Maverick’s identification.
After they see his ID, the officers pin Maverick to the ground as his three children watch. A gathering of people on the sidewalk grows, and the police officers let Maverick go with a warning.
According to Starr, the reason the officers pin Maverick to the ground after seeing his ID is because they recognize his last name. The officers must know that Starr Carter was the young girl who witnessed Khalil’s death at the hands of a fellow officer, and they are angry upon seeing the name on Maverick’s ID.
The reader can infer that the officers pin Maverick as an expression of their rage, but they also do this as a form of intimidation. They are communicating to Maverick—and thereby to Starr—that they are in power and that no amount of activism or testimony will change that.