looks like you have put so much force in your fin screws that the under layer of glass and carbon has given up. Definitely is something that involve the force used to screw your fin.
It might be also that the alligment with the screw holes and the board holes are not spot on and the screws are rubbing against the side wal of the screw holes. Many stories have been circulating in the forum that the fin box holes in many boards are not lined up and people had to file a bit of material in the holes to line them up..
I have 3 isonic boards all different sizes and one of them I had to this to be able to line up the screw with the fin hole once the fin inside the box.