What do you think happened to Mituna during the Pre-Scratch trolls' session?
I think Damara had something to do with Mituna’s burning out. There’s no concrete proof of anything regarding this, all we know about the issue is that ‘Kurloz was probably there with him’, since he was, or is, his Moirail.
Now, with the way Aranea talks about Damara, it’s obvious that while Meenah’s bullying made her snap, Kurloz influenced Damara to flip the fuck out entirely, and cemented her allegiance to Lord English. Given Kurloz was more of a silent puppeteer, if the Doom the Session was going to face was something a Troll was going to do, then it most likely had to do with Damara.
There’s also Mituna on the Part 3 of Meenahbound appearing randomly. The only reason for him to be passed out under the Brain Tree is for Latula to show up when Damara tries to get near him, and telling her that if she gets any closer she’s fucking dead (Double dead?). Either way, while yes, ha ha, Damara is deranged and a pervert, the fact Latula is there to block Damara from getting any closer in such a blunt, direct way, seems to imply to me that there’s a deeper connection between her, Latula and Mituna, which sets her as the prime suspect.
But if it wasn’t a Troll, and rather, an Event of some sort he tried to prevent, things get more complicated. The issue is to think what Doom they may have been about to face? Since, they lost the Session, they all Died, and the Scratch was performed by Damara, gleefully, because she knew that after Scratching, LE would rule over their Universe. Damara would have done nothing to jeopardise the arrival of Lord English, so it can’t have anything to do with the Scratch Construct, and they all Died in the end so Death is an irrelevant ‘Doom’ to prevent.
The only thing I can think that would fit the bill is The Tumor itself. With the Session being Unwinnable and everyone going to Die eventually, the thing Mituna could prevent that could be so important would be Complete Oblivion, disappearing entirely, forever, and instead appearing on the Dream Bubbles as ghosts. Did he maybe slow down the countdown? Captors and Programming go hand in hand. Or perhaps he saved it from the Core of the planet before the Meteors destroyed everything? Slowed down the Reckoning enough that they could extract the Tumor from the Core… If it had to do with The Tumor, it wasn’t extracting it from the core by itself, since there’s a SBURB Device made to specifically perforate through the Battlefield’s Crust, this thing:

So it would be dumb for the overexertion to be something any old SBURB Device from the Phernalia Registry can accomplish.
So there you go, those are my two primary suspects- Damara Megido messing something up, and The Tumor being destroyed.