My thought... The Ford (showing plate) was pulling the trailer and tractor, the Ford lost the trailer off of the 5th wheel hitch (maybe broken hitch or pin), the dolly pin/goose of the trailer went into the ground after coming off of the truck (trailer is now stopped dead, truck still moving) and then the "untied" tractor came of the trailer and dug the FEL into the the ground.
All in all the ford driver said "Don't worry Bill the Ford can handle the load and no reason to tie the tractor, I am only going a mile down the road".
Just another reason not to over load your truck, even if you think it can pull a house.