I'm kinda bored, but this is a topic that I'm absolutely fawning over, this is quasi-informed speculation:

I don't think there is a common failure point, from what I've seen. Easter Island collapsed from disease. Various Amerindian cultures suffered similar fates, those who made it through were victims to genocidal campaigns. Some of the earliest societies we really just don't know. Like the Indus Valley, I think it was FoC that covered it, but there's evidence that water supply fell short of demand, and they appear to have been highly dependent on it. Sometimes it's war, and war can be incredulously asymmetrical, Battle at Marathon, or the barbarians that demanded tribute from the Chinese Empire, Incans, Aztec, etc...

I think the one running theme I've pressed out of it is initiating a civilization in the first place. Yuval Noah Harari in Sapiens describes this metanarrative crafted out of a shared myth, be it gods or legal fictions. And it's true, fiat currency, corporations, ownership and et cetera are the functional mythos by which we operate. But it's a house of cards. In the real human timeline the practical concept of state/society/civilization has only existed very briefly. And a considerable amount of information we take from it is inferred, and some of the constructions underpinning our narrative are purely inductive.

And I think there's also room to debate on the definition of fail states. If there's some radical reconstitution, I think that counts as a fall. When Caesar, and then Octavian, and the transition that they ushered in, I'd consider that a failure. The various revolutions we've seen throughout history I think would count even in the case they weren't successful, excepting the American Revolution. I'd even go so far as to argue that something like the introduction of the Fed could be at least an admission of failure, and then Bretton Woods. I guess my argument is every time that house of cards falls, y'know, the social contract, falls flat - it's a fail state. I don't think that's necessarily acknowledged as such though.

I'm reticent to cartoon it. In the more literal sense: you've got conservatism which removes degrees of freedom, you have progressivism which increases them. Too much conservativism reduces dynamism, and the system can't adapt, or is maladapted. On the obverse, the civilization loses directionality and, I dunno, random walks and loses its identity (social contract) at which point maybe internal stresses build to breaking and you get a radical reconstitution. Acceleration and a sort of whiplash could explain other cases. I think there's a Goldilocks balance, which itself is sort of a moving target. But this is all relative, too. A warlike civilization will be conservative in a mode of warring, and a peaceable civilization would be progressive in adopting it.

And of course we have to meter all aspects that are within the sphere of influence of the civilization, and how impactful it is. Sort of an inverse squares problem in earlier civilizations where it was actually difficult for kings to enforce their dominion, and information was slow. The ability for a civilization's state to "self-motivate" is pretty critical for its existence (schismatic, or low-influence), otherwise it's really just for play - and I'd argue that's a pretty crazy semantic territory I can't make ground in.