Looking at the video above, it looks like the head end stayed on after the collision.
The derailment is somewhere back "in the train".
Perhaps there was something -- a piece of the wrecked truck -- that got caught up underneath?
Or... perhaps the "in-train forces" that occurred when the engineer put the brakes into emergency actually "popped a car off the rails" -- after which a few more followed.
When you "dump" the air on the head end (emergency), the brakes apply quickly, and if the front of the trains slows more quickly than the rear pushing against it, things can get dicey...