It sounds like the cracking issue is a cumulative problem. I think from the explanations by those affected might be a result of unseen micro cracks that with a hard enough impact results in broken glass. I am by no means an expert but it seems to be like a plausible explanation.
I was skeptical at first but I am now convinced that the problem does exist and only hope it doesn't happen to my phone as well. I think the best way to deal with this is to contact Motorola directly and request a resolution.