Gary, you might want to do a general google-type search under the topic rotten/mushy black walnuts and see what comes up.
You might find it has little to do with weather or wet except as they might contribute to viral or fungal issues.

If you've noticed this problem you'll likely find it discussed on the general internet more broadly than in this small website which is dependent upon members participation.

You might also look for answers from your local/regional Universities.
You can look at the other tree or nut forums here on GW but this is slow time of year for participation.