The cracks seem to occur at the lower corners of the screen/bezel junction. There is obviously some defect. Whether there is some pressure point in that area or the mold-form did not provide adequate strain relief at those 90 degree cut-outs, the cracks propagate from those areas. Also, having not read of similar issues with the white case, there may be something inherent in the composition of the graphite case's plastic or coloring agent that allows the cracking to occur.
I have read that some folks have opted to not get a replacement (refurbished unit) but to at least report the issue and note it on their account, so if it gets worse, they can follow up near the end of their warranty period.