I know what happened.
PSU Failure, the other units didn't die because \#physics
Okay, so according to OP, he was running it all off of a DPS 4kw unit.
Red flag-ish here (you shouldn't run a singular rail across multiple systems), but let's keep looking.
"3 S9's"
Um....red flag, that takes up over 3800 Watts by Bitmain's numbers....so you're running the PSU at 95.6% and some change. HUGE RED FLAG
You should never run over 80% full load capacity on any power supply for Bitcoin mining (Constant load), power supplies aren't meant to run at >90% continuously.
So that sucker was going to fail....you know how these guys fail? Voltage goes crazy, either way up, or way down. Now typically the voltage drops and most times, this can be caught by a computer, which will shut down, but not a Bitcoin Miner, no sir....you know why?
Because #efficiency, that's what. People wanted more and more on smaller boards running at lower voltages at smaller manufacturing sizes.....so, we have buck converters.
You know what they do? They jump up the voltage. Up up up!
So your power supply begins to fail, the voltage starts to drop and the power doesn't change, remember, those thirsty boys need 3825 Watts!
But wait! The lower voltage means something has to increase, and that's the amperage.
Here it comes.......12V @ 1275 = 106.25 Amps (35.41 per buck converter).....now 11V @ 1275 = 115.9 Amps (38.63 per buck converter)
Poor mr buck converter's startin' to heat up, his coils are getting toasty, singing the enamel, but it's holding.
10 volts
Crap, Mr. Buck Converter's coils are getting red hot, the enamel is burning off....HOLY...............
Poor Mr Buck converter.
The wires all short, now that 4000 W power supply just became an ARC welder, He's blowing through the amperage required to weld 1/4 inch stainless steel!!!!!
KABLOOM! (ARC Flash) consumes everything in it's path.
But what about your other miners?
Why didn't they die?
The answer is they likely experienced some damage to their coils, but the miner that died saved them, electricity follows the path of least resistance, once that one circuit board started dying, it took it all. (Replace "Missile" with "Voltage" and "Giant" with "S9"....tears man
https://www.youtube.com/watch?v=SXy3f6f9DxI)
Sucks bro, but it happens when playing with fire. I used to run stuff near 100% until I learned about the 80% rule, burned up a PSU and nearly lost my motherboard. Sucks you have to learn this way though.