Glad you asked, had been meaning to document all that.
My band failure theory is not really rooted in anything particularly scientific. And I don't know how bands typically fail so maybe mine failed in the typical way....although I would've expected it to just wear down over time rather than having large sections of the friction material just rip clean from the band. Honestly, the most likely thing is that it was a manufacturing defect in the band. I read in another thread where somebody said that water can break down the glues used in some of these parts, but I've never had any evidence of water in the transmission. The low/reverse band seems to be fine, as are the clutches, and I would expect that sort of cause to be present throughout.
But I'm also suspicious of coincidences. About a year ago I dropped the transmission to do the vent relocation. This involved 3 main relevant steps, the first being drilling the case for the new vent. I took great care in drilling the case (detailed in a How To thread on this site), using greased cardstock to catch the shavings from when we drilled the hole. You could see where they all landed in the grease, it was a normal distribution with a big pile in the center and less shavings as you moved away from center. There was nothing more than 2" from center, so I'm confident we caught 100% of it. This was done pretty much directly over the kickdown band (which ultimately failed).
The second potential cause step was to fill the exiting breather hole on the pump. I did this by tapping the existing hole (it was already the correct size for the tap). What I did not do was open the pump and catch the shavings created by the tap. Was this a bush league move? 100%. At the time I was already in over my head in even pulling a transmission, much less removing the pump & doing band adjustments to hold the drum in place. Turns out opening the pump halves is ridiculously simple (literally just unbolt it) so it was dumb of me to not do that. Regardless, it's likely some shavings from that tap found there way into the trans. Of course the pump distributes ATF to all parts of the transmission, not just the first band, so there's no reason to think that my bad judgement a year ago is responsible for this, but like I said I'm suspicious of coincidences.
The final potential cause step is that it's recommended to tighten the first band to hold the drum in place when you pull the pump. Otherwise the drum can sort of fall to the side & possibly cause the thrust washers to come out of their seat. First time I did it I was scared to do the band adjustment, so I didn't. Working horizontally, the drum did kindof fall down a little bit. We tilted it vertical to re-install the pump and had no issues there. I got lucky that the thrust washers didn't unseat. But I ended up pulling the pump again, and this time I did tighten the band adjuster. Repeated the same process, although I did need to loosen the band once vertical to get the pump to seat. I wasn't convinced that wasn't also coincidental though (says the guy who doesn't trust coincidences). To avoid somebody asking, I did follow the FSM process for re-setting the kickdown band adjustment
to the letter.
Then I drove it for a year without any issue whatsoever until it decided to act up completely out of the blue, without any signs of degraded performance leading up to it.
TL;DR- I drilled into the transmission in 2 spots, both very close to the section that failed, and I also touched the band adjustment screw on the band that failed, and did so multiple times in the process.
Occam's razor would suggest that none of this is related, and the band just failed. I mean, would any of the above take a year to manifest itself, without presenting symptoms along the way? it feels like whatever secures the friction stuff to the band just gave up the ghost. Why it would do that is anybody's guess.
Conspiracy theory would suggest that where there's smoke, there is usually fire, and I somehow f'd it all up when I had my grubby hands all in there. What set of actions led to the failure is anybody's guess.
I believe
@U8MYDZT did the same vent relocation during his rebuild, would be interested to hear his process to see if there was any major difference in our methods.