Why and how often do AI Empires change their names?

You don't give a lot of information to work with, but I'll enumerate the possibilities I've seen so far:

• Change in Government, which can be instigated by changes in Authority, Ethics, or Civics. ( I know you've ruled out ethics changes, but policy changes should be irrelevant.)
  • Ethics changes are made by embracing the related Faction in the empire.
  • Authority and Civic changes are enacted at a cost of 250 influence. See the "Reform Government" button to do this in your own empire. Authority changes can be used to appease Authoritarian or Egalitarian factions and are far more likely to result in a change of Government than the other two factors.

• An empire splits due to Rebellion (generally a single planet) or Liberation (generally a handful of planets). The new state gets a new name, often similar to the parent state's.

  • Rebel States often subjugate themselves to alternative (preferably Egalitarian) overlords (only to be integrated 10 years later).
  • Liberated States embrace the Ethics of their Liberators, are very likely to ally themselves to their Liberators, and are likely to last longer.

  In either case the parent state still suffers from the same internal/external pressures that created the original split, and may not survive long.