It was probably not caused by trimming the quilt incorrectly. That might happen on one corner, but is unlikely on three corners.
The only suggestion I have is what others have suggested. If you have enough of it, take some back to the store you bought the fabric from and ask for your money back. It would help if you still have the receipt. You have nothing to lose by asking. You may not get all your money back, but even an adjustment would be nice at this point.