My take: it's the reaction by Clans which clung to fundamentalist Honour principles, who kept losing because "the other side cheated!" (eg. did not play to the fundamentalists' advantages) - who then reached the point of "I'm mad as hell and I'm not taking it any more!"
So they went "over the line" and basically threw Zell out the window. The purpose of fighting is to win, and you win by application of superior force. All of a sudden, the IS could no longer exploit Clan behavioural blind spots. No bidding, no duelling, just killing.
They also took the line from the historical Mongols - you got one chance. Surrender, and you will live under our heel. Resist, and you - and your family, your city, your nation - will be exterminated to the last child.
While this started with the Hell's Horses, the bulk of the Jade Falcons embraced this enthusiastically. Many other Clan honour-based traditions dropped away - use of combined arms, physical attacks with 'Mechs, dropping WarShips on your opposition, little things like that. Malvina threw herself out in front of the curve, and many Falcons saw her winning. And "Might makes Right", quiaff? So all that pent-up frustration got harnessed by a sociopath. As they used to say oin the TV guide, "hilarity ensues ..."
And eventually, even the Hell's Horses got a bit "Whoah, dudess, doncha think you're taking this a little over the top?" and broke the bestie relationship. Doesn't mean they're not Mongol any more, just Mongol Lite ;)
Others' opinions may vary.