After America would be like the Fall of the Roman Empire, or the collapse of the Chinese Jin (romance of the three kingdoms) and Tang (five dynasties, ten kingdoms) eras, usually because of human bickering over power and control. Occasionally, systems like Shadowrun have a "mild" apocalypse that mostly serves as a catalyst for balkanization. Whatever vestiges of a state remained fall apart under the stress.

Complete apocalypse tends to be something like large scale devastation from a known threat that final gets used (nuclear, biological, dangerous machine sentience) and everybody's too busy dealing with their own issues to care about larger ideas like a continental federal state of "America."

Either way, tends to result in 3+ most of the time. From looking at the Roman Empire and the multiple collapses of China though, it really does not take anything especially dramatic to result in pretty severe balkanization. Often its the old "Blue and the Grey" divide and then most of the West just does their own thing. Occasionally it's more like East Coast, Heartland, and often the West still is not really included.

The result for the West has actually been one of the weirder parts of reading a lot of those settings. Often this undercurrent that the West has never really been a part of "America." The heavily populated East is still mostly fighting over the same issues with each other, the lightly populated West is just some far away land they occasionally pay attention to (mostly California and Texas).

Civil wars and the like are usually based on youth bulges, as they need a lot of breathing bodies to fight it out. Preferrably slightly hungry bodies, as hungry people are easier to provoke into fighting.