Watching the helicopter fly-by, the crash scene is huge. No doubt the rig was travelling extremely fast. You don't get that kind of carnage from a typical t-bone accident between two cars at an intersection.
The corners show some stop signs too, but hard to see if all of them have one of if some of them were knocked down, but you can see at least one stop sign still up in tact on one of the corners.
So either the trucker blew through a stop sign at highway speed, or perhaps his side has no stop signs (right of way), while the bus driver's road had stop signs but he didn't stop.
Either way, some careless driving from at least one side. The intersection is a quiet looking country road with good visibility. You should be able to see what vehicles are driving perpendicular to you by glancing left or right.