I do not have your choices of "the following" Here are some reasons I learned: Christianity, empire was too big and became hard to manage, there were many nonromans attacking parts of empire- Goths, the emperors themselves were weak or insane intrigue was rampant.
Drying war, civil liberties usually become more restrictive. In WW1 there was the Espionage Act and Sedition Act which undermined the first amendment.
