I don't know, the original video only catches the last few seconds before the SUV hits the first bike. Watch it again, it looks like the SUV is
VERY close to the bike, right on his butt, and that is where we start seeing the video....hard to say how long he was doing that....at one point he is riding up along side the bike and squeezing him in the lane.....and the bike starts to slow down and looks back at the SUV driver.
If the SUV driver was tail gaiting the bike dangerously close (and it looks like he was) it is not at all unheard of for the one being tailgated to slow down slightly (in Georgia that happens a LOT on rural highways as a way of telling the guy behind you to "get off my tail now or I will go slower!").
Watch the beginning of the original video again....I think maybe there was a problem BEFORE the video clip showing the SUV hitting the bike. I can also say if I were on a highway around that many bikes I would be moving a good 5 or 10 miles slower than they were to let them pass me I would NOT tail gate any of them (they can hit the back of my vehicle but I wouldn't risk hitting them).
https://www.youtube.com/watch?v=F4tgN2glwd8