Put the three facts together and the pattern is obvious once you see it. Charlotte's housing stock in this service area is concentrated in a 1967-2011 window, with most of it dating to the 2000s across the named neighborhoods and towns we cover - meaning a huge share of doors are hitting spring end-of-life at roughly the same time, not spread evenly across decades like you'd see in an older or newer market. Layer on a cold-moderate climate that still delivers enough freeze-thaw swing to stress fatigued steel, and you get failures clustering in late fall and early spring rather than randomly through the year.
This isn't a Charlotte-specific defect or a builder shortcut - it's just what happens when a lot of the same-era hardware ages out under the same weather pattern at the same time. A house in Dilworth built in 1998 and one in Cotswold built in 2004 are running the same spring math on slightly different clocks. That's why we see the same failure mode call after call in this area, rather than the scattered mix of issues you'd expect somewhere with a more varied build history.