Body flex
Install a Strut brace
Both of my cars have windshield cracks, with the instalation of the strut brace the cracking stoped. I've replaced the windshield on that car, with no cracking. The car with no brace continues to crack.

2012 Lotus Evora --------- 95 Mystique MTX - GONE 395,000 Kms --------- Both Zetecs '97 Contour Mtx - stock - GONE