The corruption of the @Weepingwood stems from the @Abyssal Seed, which comes from the Shadowsea and is a source of an abyssal fungal infestation that is tainting and warping the forest life. Creatures like @Shroommutts, @Bloom Horrors, and @Carrion Sporedusters are twisted by the parasitic spores that are growing and spreading from the @The Withered Hollow, which is the center of the abyssal influence.
There should be clues throughout the surrounding area that eventually lead to the discovery of the @The Withered Hollow and the source of they dying @Weepingwood.