The official explanation so far is that the capsule was secured in its holding device which in turn was in a box secured on a pallet. The device was shaken apart by road vibration, and likewise one of the four bolts holding the box to the pallet was shaken out. The capsule bounced through the bolt hole, off the back of the truck and onto the roadside.
I think we'll have to wait until the Australian authorities finish an investigation to find out whether that improbable sequence of events is what happened, or whether Rio Tinto put a badly damaged, disassembled detector assembly into a box, did a poor job of securing the box, and THEN the part bounced out from road vibration. Given where it was found, though, I think we can be reasonably certain the last part is true.