Faulty grade and pest infestation
Having the foundation a reasonable distance above adjacent grade is an important aspect of deterring termites (if that is your problem). It will not stop them, but it will make access much more difficult for them. Having wood at or below grade is like leaving your front door open with a big sign on the wall saying "Come on in! Take all you want!"

One thing to maybe consider, especially if you are replacing wood anyway, is to replace all wood too close to grade with pressure treated material that meets the standard for All Weather Wood Foundations. It specifies higher preservative content than typically seen in material available at big home improvement stores, as well as specific standards for the use of corrosion resistant fasteners.