Watching the video in slow motion it does appear that the motorbike driver may be the primary cause of this accident.
When he first enters the screen he seems to be braking quite heavily and definitely appears to be coming from the right lane, or in between the lanes, heading for the layby. It certainly does not appear that he was driving in the left lane before he started to pull over.
With this in mind it seems possible that he raced up in between the lanes and then turned left and braked without checking what was behind him. If he did indeed try to go directly from the right lane or in between the lanes without thoroughly checking his left mirror and over his shoulder then I think it's his fault, particularly if he did not first gradually move into the left lane well before attempting to pull over and instead tried to jump directly into the layby from a central position on the dual carriageway.
The first comment on the page you link to seems to support this...
Quote:
I was actually driving behind the bike when the accident happened. Although rather an unfortunate incident, the motorbike was also travelling at high speeds and undertaking various cars in the lead up to this accident.
Obviously, without witnessing the event in person or from a camera further back it is impossible to tell what really caused this accident. But I certainly wouldn't immediately assume the car driver was tailgaiting and is to blame.