Why did Dana Matherson grab Roy around the neck on the bus?

In Carl Hiaasen's novel Hoot, Dana Matherson is a bully at Trace Middle School, which is protagonist Roy Eberhardt's new school. Dana and Roy ride the bus together. Dana grabs Roy on the bus on page one of the novel, but not by the neck. He grabs his head and presses his thumbs into Roy's temple. This is the inciting incident to the conflict between Dana and Roy that continues throughout the novel and runs parallel to another conflict—the plight of the burrowing owls. Here is the quote:

But on this day, a Monday (Roy would never forget), Dana Matherson grabbed Roy's head from behind and pressed his thumbs into Roy's temple, as if he were squeezing a soccer ball.

The reason Dana does this is simply that Roy is new and Dana is a bully. He had no other reason to do it—he and Roy had never interacted before. Later, in chapter 9, Dana grabs Roy and pulls him into the janitor's closet, but doesn't grab him by the neck.
In an altercation in chapter thirteen, Roy is tackled by Dana, but again, not grabbed by the neck. Hope this helps!