Reverting land is defined as an area that was once agricultural, but now lies fallow and trees are beginning to grow back.  There are shrubs and small trees that control runoff and erosion.  However, reverting land often has residual herbicides, pesticides, fertilizers, and manure that can get into the groundwater or can be carried into the lake during storms.
When the land was left fallow in 1965, it was probably allowed to grow into forest, but more recently, some of the reverting land may have been converted into commercial or residential areas.  The decrease in reverting land is advantageous to the lake.  It means that the most of the land that was once identified as reverting has now become forested over the 38 years.  The higher density of trees helps control erosion and runoff while absorbing many of the nutrients that were left in the ground from the fertilizers and manure, keeping nutrients from reaching the lake.