When something like this happens, it always takes a few days for the facts to solidify. In the initial reports, I'd read he reached through the window and either hit the officer or was grabbing for his gun.
It would certainly explain the 'tussling through the window', as this lady witnessed.
Witnesses say that the cop initiated the physical contact by reaching out the window with his left arm and grabbing the kid by the neck.
The kid then tried to get the cop’s hands off of him and back away — hence the tussle at the window.