There is a cause and it's not post mint damage, at least if I recall correctly. If memory serves me there was a CoinWorld article some years ago that addressed this, but I forget the details.
The more I think about this, I believe the mentioned article may have addressed this sort of thing when it occurs on the rim as opposed to the field. Sorry that I can be of no greater assistance.
I think I read that .. looking there's also TRAILS ==> https://www.coinworld.com/news/us-c...stery-surrounds-die-trails-inside-cw.all.html more info http://www.lincolncentresource.com/TrailsandWavySteps.html
No... that's not it. This had to do with lettering appearing on the rim and has something to do with striking. The article would almost certainly have to have been no later than 2012. Thanks for trying though. Much appreciated.