Shaken Comic Syndrome. Excessive shifting of the comic inside the case that causes it to get bumped/bruised by the plastic itself. Sometimes happens during shipping.
Since you're not the original submitter of these books, I doubt CGC will entertain much discussion, as they cannot vouch for any handling after it has left their custody.
Just to clarify - I am not suggesting that you "bought the label" on these comics (though that's much easier to do online), but that you should keep them based on how they present after you received them. If you don't think they look like the grade you bought them as, returning them seems like a no-brainer. If you haven't read/heard that term before, though, keep it in mind!