[JRTJH]

Photos would be a tremendous help. There are two vents, depending on which vent (upper or lower) the rot could be caused by different types of leaks. In the lower vent, the most common cause of "wood rot" is a leaking drain line, either broken (it's cheap plastic and deteriorates from the heat of the refrigerator chimney) or that same drain not being placed through a hole in the lower vent, so it drips on the wood inside and causes it to rot.

When you post your photos, we'll be able to suggest a cause and a solution.