Could be a lot of things, like watering strategy, weather, plant location, soil, etc, etc, but my guess would be cross-pollination. Heat seems to be one of the first traits to diminish/disappear in crosses, at least to me. Of course that does depend on what crossed. Is crossing a possibility?

Or, they could be a hybrid, rather than true Carolina Reapers. If you got the pods from someone else, maybe they grew other kinds of peppers with it.