As your body sheds fat, it no longer needs to expend energy building, preserving, and keeping your fatty tissue warm. JIT energy use instead of lossy battery storage. It doesn't further slow your metabolism beyond what you are genetically predisposed towards, but metabolism does slow (a little bit). Another way to think of this is, when you consume in excess of what you need for healthy living, your body begins consuming additional calories in order to build and support the additional long-term reserves you are creating and maintaining. We have decades of research supporting this notion (as well as common sense).
There may also be an aspect of "hibernation" that kicks in when dramatic caloric restriction is suddenly introduced. That is, you are further inclined to be less active as you lose weight quickly. I'm less certain of that, though.