The 'ducking' reaction is caused by the fragments from the headshot.
Headshot - fragments - reaction
Not to sidetrack this too much but I have always been confused by Kellermann's usage of the phrase "flurry of shells come into the car." Later, he said "flurry of shots." Shells or shots or what?
When asked for more details, he said this:
Mr. SPECTER. Now, in your prior testimony you described a flurry of shells into the car. How many shots did you hear after the first noise which you described as sounding like a firecracker?
Mr. KELLERMAN. Mr. Specter, these shells came in all together.
Mr. SPECTER. Are you able to say how many you heard?
Mr. KELLERMAN. I am going to say two, and it was like a double bang--bang, bang.
Two shots or shells, for me, is not a "flurry" of anything.
I would guess that the sounds of the fragments hitting the chrome and the windshield must have sounded like shots. Or shells. Or something. But he said just two "shells."
In any case, as I've said above, trying to discern all of this from the accounts will drive you mad. It's frustrating as hell and I gave up on it a long time ago.