Unfortunately, we cannot provide a direct answer to that question. Medicare Part D Drug Plans can change their premiums every year and some Part D premiums have gone up every year - as in your case, and other Part D plans have actually reduced their monthly premiums. Actually, monthly premium changes varied across the different states or regions.
Most likely, your Part D plan experienced a high loss or claim rate in your area causing them to raise prices so as to meet the increased costs. We notice that in some states, the number of prescription drug users is simply higher or increasing as compared to other states. Often times we see that a Part D plan with lower premiums one year will attract so many people with high prescription drug use that they will need to raise prices the next year to bring the plan back into profitability.
Please also note that Part D plans can only raise their premiums or change structure with the approval of Medicare.