Whilst nobody here can explain the potential “trigger”, taking your description at face value, you may have seen the result of a stress fracture failure. Localised heating or cooling (perhaps from internal components - such as during charging) can be a trigger factor.
Glass is often deliberately pre-stressed during manufacture to make it (somewhat ironically) more resistant to breakage - and hardening of the glass can often introduce additional stresses. The Home Button cut-out is an obvious potential “weakened” area of the glass panel The slightest imperfection in a glass edge (or minor impact to an edge) can, occasionally, lead to a fracture failure.
Unfortunately, latent manufacturing defects of this nature in iPad screens appear to be relatively rare - and when failure occurs, it is almost impossible to attribute such failure to a latent defect. Unless this is an issue regularly seen with a particular product, where a generic or batch defect might eventually be inferred, you are very unlikely to persuade a retail outlet that the issue did not occur through accident or mistreatment. It the device casing has other damage, attribution to misuse is easy to imply - but the converse is, unfortunately, not the case.
In absence of accepting “bad luck” (sometimes “things” beyond our control do happen), you might consider the fall-back option of claiming upon a relevant insurance policy if you have one. Your Home Insurance policy might provide cover for “accidental damage” - but your excess/deductible payment (and subsequent premium loading) might make this route equally unpalatable. Otherwise, if you are sure of your position - and are prepared to politely persist in presenting your case to your retailer (sometimes the Manager can be politely brought on-side) - there is little else to try.
I hope this reply is, in some way, helpful to your plight.
