You are right about the U -turn which was closed a few years ago after a Farang got wiped out. That lane is in constant use and (in day time) it is very much a fast lane.
There is only one root cause - the motorbike rear-ended the truck. I am not guessing I have seen trucks make that turn many times. The truck would not turn from lane 4.....you can see from the truck's position exactly what happened. This looks like an early hours delivery, quiet road, if the biker could not see, or stop in time for a huge container truck then his his judgement was flawed by any number of the factors I mentioned earlier. A toxicology report would be the next thing to see.