They stop innovating. It's that simple.
Whatever causes them to stop innovating, when they stop innovating they stop solving problems their ancestors easily surmounted and these problems begin to pile up.
Maybe they stop innovating because their institutions are seized by sociopaths. Maybe their entire culture develops into a worship of the lower class, ghetto vernacular and conduct. Their values are turned upside down for some reason. Maybe they just plain go nuts. In the modern West, all these things have happened and many more.
Ours is not a society that can solve it's problems anymore. Nothing else is important if that is the case. There is no remedy for this ill.