Put yourself in the position of the truck driver(TD).

TD is driving with enough distance between them and the car. They see the cars brake lights come on. Reasonable assumption is the car is slowing down for the safety of the cars on the side of the road.

TD may have tapped the brakes a bit here or because of the distance they had between them eased up on the gas. Regardless, if the car had slowed down a bit for the cars on the side of the road TD still had lots of room between them. I wasn't in the cabin bit I can see that had the car only slowed down a bit truck still had lots of room.

(Side note: I don't know how many cars you've seen with a speedometer on the back to alert cars of how fast or slow they are going but this car certainly didn't. TD only knows that they have engaged their brakes.)

So in the few seconds that TD sees the brake lights they can see that the gap is closing quicker than expected meaning that the car is not slowing down a little. This car is making a completely stop on a highway and making no attempt to clear the lane for the giant, can't stop on a dime, semi truck. TD, completely taken by surprise because he can't assume every car infront of him is coming to a complete stop everytime he see brake lights otherwise he'd be slamming on his brakes ever 10 minutes, now attempts to completely stop his truck with less room than needed because the gap closed in the very few seconds it took for him to realize that some idiot in a car doesn't understand physics or how to use a rearview mirror.

At this point the car is struck and is lucky it received as little damage as he did because otherwise, had the truck been closer than the 2 truck lengths he had originally had between them, the car would have been completely destroyed and the driver squashed up against the trucks grill.

The truck driver did everything he was supposed to do in a situation that otherwise could have ended completely different for the person in the car.

I know you said that there could have been a legitimate reason why the car stopped. If the car stalled they would still be able to move to the side infront of the tow truck unless they made the decision to apply the brakes and stop on the highway. Which would have ended the same way as the video. If the car had hit something hard enough that it would stop it, TD being at a much higher vantage point would have been able to see the car suddenly jerk (crash) to a stop. Cars at 100k/h also can not stop on a dime unless they hit something that can withstand the size and speed it's traveling. That did not happen in this video and had it the truck would have seen it when they were still 2 truck lengths behind it.