Incorrect pinion depth setting, I once ruined a rear end by not crushing the collar correctly. It was not in a semi though.

That's an easy one. Take a look at the damage on the ring gear. No where near where the load is supposed to be.
Something got distorted or out of whack to the point were the load was no longer transferred to the center of the gear where it belonged, instead it was transferred to the edges of the teeth where it shouldn't be.

The failure did not start with the gears, they are the result and not the cause.
My bet is either a bearing let go or some other mechanical failure which resulted in a significant misalignment of the gear mesh.

The rear tandem lost traction while the front took the full force of the load. The drivetrain wound up, unwound, would up again then BLAMO!

Wow - you guys are good! Two camps here: 1) Driver Abuse 2) Bad Set-up

Very good! Now, if you believe it is one, prove the other wrong and if two wrong prove the one wrong!

I am of the belief that it is both 1 & 2! (but not the gear oil!!!)

Really hard to tell from the pictures.
I've seen some idiots trying to slide their trailer tandems and it's a wonder the driveshaft doesn't get bent like a pretzel let alone what it does to the rear end and U joints.
I've never had a rear end fail. I replaced mine(2 years ago) at over a million miles because the nose bearing had loosened up enough (frt. rear) that i couldn't keep a seal in it. I wanted to change the ratio anyway so i swaped out both rears at the same time.
If there was no flaw in the pinion i still say driver abuse.

Kind of like aircraft accidents and fire investigations, I'm right because I said so and it's always pilot error.

Naw, I wouldn't say driver abuse, I'd say rookie mistake.

He who has the best technical rhetoric wins :D

+1.....This is also my vote
And I agree with Sammy (rare) that the gear wear pattern is the result, not the cause.

Bad lubrication? :)

Wheel hop and other driver mistakes usually damage the axle shafts, not the R&P.
Now if an axle snapped and loaded the diff with debris, you could have significant R&P damage.

If the axles are not damaged, I say to check the R&P setup. Either it was a bad setup, or as speculated in another post, a pinion nut or bearing failed and allowed the pinion to drift out of proper mesh.

Those are my guesses.