1. succession practices divided the empire, and subsequent kingdoms into parts for all surviving sons +
2. empire reasonably ceased it's expansion policies, leading to
3. ambitious lords turn to internal conflicts to increase their holdings.
4. arguing over imperial title from the kings.
5. short-lived kings and emperors.
6. Vikings, Arabs, and Magyars raiding and inability of the centre to respond.
7. lots of small scale local conflicts means local leadership is needed as center is too overtaxed, forcing delegation of power to locals, leading to.
8. local entrenchment.

As far as I know what was at first Charlemagne's bureaucrats became local lords. Eg count being a position the king was supposed to be free to give and take becoming de facto hereditary.

Early Medieval Europe 300 to 1000 by Roger Collins