Most likely, you were shooting the guy and he wasn't dying. When your driver rammed him, it had already registered you as killing him, but the lag was crazy and gave you the kill like it should have, because you shot him.
It looked like the guy was crouching behind something. Maybe you shot him at first, but then, because of the lag, for him the boxes spawned as cover for him. So he was hurt, but behind cover. You kept shooting the boxes until Dthen hit the "boxes" that he could see, and sice he was still damaged by you, the boxes splattering him (not Dthen) counted as your kill. At least, that's how I see it.
I agree... that seems like a pretty good guess which I would probably never think of. Oh and err... nice Purple Cubes banner.