Here is a 3rd possibility:
The truck was driving in the #2 lane, where you would normally drive. In some states you must not drive in the #1 lane... this is for passing.
The bikers either were in the #2 lane, or halfway.
Truck coming up on them at 75mph doesn't realize this is a slow moving vehicle. Swerves at the last moment, clipping the left half of the group.
smacking the left rear of the Subaru with the right front of the truck.
I don't think we will know until they have questioned everybody. And most likely they have asked everyone not to comment.
That gravel shoulder.. looks like redrock gravel, or the worst chipseal you ever saw. Which is it?