My theory is that someone copied my artwork, separated them from any typography and then posted them for sale on the stock site. Someone working for the site either saw my [LogoPond] showcase or was alerted to the similarities. They then prepared the bill and sent it to me. The good thing is that the bill gives me a record of every single image they took from me. That helps me gather dates, sketches, emails, etc to help me prove my case. The bad thing is that despite my explanations and proof, they will not let this go.
No man is an island if he's on at least one mailing list.