From what I can gather watching the video and having participated in large rides like this...
To start, it seems the SUV was determined to drive in the middle of the road. GENERALLY, the etiquette when there is a large group of bikes on a multilane road is to get into the right lane (unless you need a left exit for some reason) and give the outer lanes to the group of bikes. That said, it's etiquette, not law. It seems that despite being in the midst of the group of bikes, the SUV was simply wanting to ride in the middle lane and probably had been for a bit of time as the bikes ended up "Surrounding" it because they had to drive/pass on both sides because of where the SUV was driving.
It then looks like a few bikers become annoyed that the SUV is doing it, and one particular idiot seems to get in front of the SUV and preceeds to break heavily to slow themselves leading to what LOOKS like the SUV hitting the bike from behind (we don't see the contact on the video, but right up to the moment before it).
This contact would actually provide a reason why all the bikers likely finally stopped. While people in a group may be annoyed by someone with poor etiquette, the moment a bike is actually HIT (For whatever reason) it's likely to draw the attention of the entire group. Once it was realized a biker was hit, the entire group likely came to a stop to both give the hit biker a chance to regain control of the bike AND likely (wrongly) to give the SUV crap.
Now, while I think the hitting of the biker is the REASON everyone stopped...the stopping isn't really justified nor was the hit entirely on the SUV guy. At this point your probably having bikers talking/arguing with the SUV driver, causing him to be nervous, and leading to the whole mess that proceeds to occur.