I gotta agree with Whunter here, that is a stress break, and it's on the bolt hole, so the two ends of the bracket are pivoting. something on your bracket is moving! if you can get the car up on a lift, and get some pictures of the underside I may be able to assist your diagnosis. if not, eventually I will have my 87TD on a lift and I can see what could be causing it to stress fail.
look carefully at the picture. you will see wear marks on one side of the bolt hole from the bolt head, but none on the other! it's moving!