If you look from this angle, it appears that he first failed to set the tractor parking brake, and then failed to pull the hand valve.
When he supplied air to the rear trailer, he should have done the service side first and noted that no air set the rear trailer brake.
Instead, he either failed to notice and go back to set the hand valve, or supplied the emergency side first.
With air supplied to all the trailers, the rig rolled backwards. You can only back a set a few feet before it will start to jackknife like that.
We can all learn from this to slow down and think about what we're doing.
R.I.P. driver