From the skid marks it does look like the truck crossed over the center line. Overcast sky so sun should not have been a factor. It could go either way, although I've seen a lot of impatient jackasses on bikes pass on a double yellow line.
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?
Those pictures are amazingly grim; obviously taken prior to emergency response. I see scattered bikes, but no riders, or injured people, or uninjured people trying to help injured people. I’ve seen many accident scenes and understand the inexplicable nature of some physical encounters, but the images depicted seem so unexpectedly final. So very sad.