The correct answer is - its army was becoming less loyal to the emperor.
After the Pax Romana, Rome faced huge problems by being constantly under attack by multiple people outside its borders. The problem arose because of two reasons, the first one being that some of the influential people in Rome didn't really cared about the well being of the empire but only for themselves, and that the army was becoming less and less loyal to the emperor, which was also a result of the first reason.
The Pax Romana was a period that lasted from 27 BC to 180 AD, and it was a period of peace in the empire, which in that period and circumstances was an incredible achievement.
Once this period ended, the Roman Empire started to weaken, and big contributors to that were the Huns, Vandals, and the multiple Gothic tribes.