I believe that it's related to the overall current draw on the system. If you exceed a certain threshold, then the ICE will run to supply the necessary power.
I’ve always gone with the drop-in bedliners in the past, but came across the Dualliner when searching for one as Ford apparently no longer offers a drop-in. It does seem like a better solution, so I ordered one. The only problem is that they don’t have the mats and they are back ordered for...