In this specific instance the problem is this guy. Roundabouts are tough on large trucks since they basically need them to be empty to enter safely. A dedicated lane for left turners would solve this.

His opening is the opening that allows 3+ cars to enter in the adjacent lane, he was probably waiting a while for it

You can also see by the time he clears the roundabout that 3 or 4 cars have made left turns in the interim, meaning the jam is already starting to break up