That’s because another truck (behind you) radio’d ahead and told them about an obnoxious driver that probably cut them so. So they block the traffic by passing really slow which inconveniences the asshat and consequently everyone else.
It's because the trucks are governed, many within a couple of mph of each other. The drivers get paid by the mile, and they have strict time limits to make their daily wage.
Some of these guys just put their foot on the floor and don't worry about it.