If you received tax subsidies in 2015 for your Marketplace plan, the tax subsidies are based on what is known as the benchmark plan. The benchmark plan itself can differ from county to county. For 2015, there are many new plans available for sale in the federal Marketplace. This means that the second lowest silver plan from 2015 offered in your area may NOT be the second lowest silver plan available for sale in 2016, and your subsidy amount could be lowered.
The map below shows how auto-renewing may affect you, by county.