I am digging up red nordland potatoes and some of them are mushy!! Why is that?
Perhaps you found the seed potatoes you originally planted. They are always mushy when they come up.
Here is a link that might be useful: My potatoes and garden
Hopefully you did not plant whole potatoes, so I expect that you are experiencing bacterial soft rot (Erwinia carotovora subsp. carotovora). It is pretty widespread, but really manifests itself under certain growing conditions.
Here is a link that might be useful: Soft rot