RH - I re-watched it and I can honestly say I have no idea what that could be. Pure speculative guess would be a smaller chunk passing behind the main mass. The angle of the view makes it look like it hit it, but it could have easily been in front of or behind. Air density increases the closer to the ground you get, so the main body would naturally "slow" a bit and anything trailing behind could plausibly play "catch up".

All I do know for certain is that nothing man made hit it. Experimental Rail guns shoot at a paltry 5K+ mph. NASA scramjets at 7.5K. Particle accelerators are the only thing we have that can move that fast (roughly 3 meters second less than the speed of light!).