Truck was looking to change lanes to the right and abandoned it. It's happened to me - check mirrors, see vehicle a long way back (so effectively clear and safe for lane change) but expecting it to be traveling at normal highway speeds. Begin lane change and then check mirrors again during lane change (to ensure nobody darted into the lane and is trying to pass..happens all the time) and then suddenly realize aforementioned vehicle is traveling 50% (or more) over speed limit and it's closing ridiculously fast - at that point in an effort to avoid an accident (even though it wouldn't be my fault) a pro instinctively abandons the lane change.
However, rider over anticipated the free lane and had already committed to it. Passing on right made it worse. Traveling stupid fast made it even worse.
Poor decision making on behalf of the rider, on so many levels. End of story.
Over anticipating a free lane causes accidents routinely, although many are minor paint scuff/body damage ones. If I had $1 for every time I was changing lanes (again, I drive a semi) from the middle to the right and the car behind me started passing before I was even 50% out of the lane I'd be able to book a nice vacation somewhere warm every year. Problem is if I need to abandon that lane change and swerve back into the middle lane (again, because sometimes some doosh is trying to rush past me on the right, god forbid they wait the 10 seconds for me to get over) suddenly the other guy who's now in my lane I haven't yet vacated gets a nice tire rubbing. That's the reason behind the law that states two vehicles are never to occupy the same lane at the same time, but like many laws it's lost on the impatient.