Ghosted image on polyester shirts from SanMar..

We recently printed a job of 150 pieces on 100% polyester Gold SportTek shirts from SanMar. After our customer received the order, they notified us of a ghosted image of the imprint on the front of the shirts.

The job was a 7 color, based design on the full back. Any ideas as to how or why this happened? Defect in the shirts? We're out of ideas..