Not at all uncommon, and they can break at the most puzzling times. We had this happen during testing of these machines (both NH and Case) and various door (curvature, for lack of a better word) designs were attempted. Essentially, the curve of the door does not exactly match the mating curve of the cab frame. This is so that the door, when closed and latched, is stressed to avoid that annoying air leaking sound at the top and bottom of the door, the farthest location from the latch. I'm not an expert in cab and glass design, but to the extent to which I was involved, only thing we could come up with was that the glass was subject to some kind of fatigue. Final approved design was a compromise between longevity and effective sealing. Dirty cab air filters do make the doors difficult to close, and may worsen the problem by forcing operators to really slam the door (excessively) hard.