Inflation. At the end of the first century rome's coins where 95% silver. In 117 AD the coins where only 85% silver. In 180 AD the coins where only 75% silver. In Septicemic' the coins dropped to 60%. Caracalla evened it off at 50/50. They where 50% bronze and 50% silver. People did not like this so they got mad and started to rebel against the Roman empire.