Wow. It wouldn’t surprise me if it had something to do with that big empty trailer. Would an empty trailer be more apt to start moving side to side - and then putting the entire rig out of control?
“The crash happened about 500 feet from lodging where most of the motorcyclists were staying, Hamanne said.”
Bikers not quite up to speed yet, perhaps waiting for others to leave the B&B, truck slams into the rear of them? Bikers turn in front of the truck, truck loses control and slams into the others down the road?
Combined with the possible difficulties of driving with a long trailer - a series of unfortunate events culminated in a disaster.
I believe that is a fifth wheel trailor being pulled by the pickup.