Answer:
Corruption; as people governing were benefiting but not giving people the protection, public services and justice in return.
loss of trust to State authority, is the main reason.
The others are military and politic struggles. Inside the Empire and outside. People were trying to gain power, and following personal interests and this caused in long term disconnections between the top it self and towards the population.
At the end only value that kept them together was religion, and even that was shredded into pieces. Catholics, orthodox and Protestants were not getting on well.
