Answer:
The end of the Pax Romana came from the fact that military power, despite being the cornerstone of Roman civilization, was not inherently tied with the power of the emperor. The Pax Romana came from the fact that people had faith in the emperors' respect towards this relationship. When it wasn't, the political succession was broken and after repeated rebellions and discontinuities, the fragile trust the Emperor had had with the rest of the empire broke down.