Hog,
Were there any property damage?
The only reason I can think of that they would investigate passed the original accident is if there was state (provincial) property damage or damage to something else other than the truck and a liability issue was raised. Then they will do an inspection on the condition of the truck, its braking system, the trailer braking system and the tires.
If all of the slack adjusters (brake actuators) were adjusted properly and not out of adjustment, then they are probably having to wait for procedural findings, which can be a pain.
The other thought is that the was hazardous material spilled (fuel or whatever), then they'd have to determine how much was spilled, whether it made its way to a watershed (fish habitat) and what it would take to clean it up.
I take it they are in B.C. or Yukon?
Are they on the AlCan Highway or the South Klondike Highway?