There’s not much clearance for those tabs. I would say it’s climate related. It would not take much. No humidity control is perfect, of course, it will usually just reduce the severity of the change. If they are in an enclosed practice room, it’s even more likely to be humidity related. Also, if they were manufactured around the same time, it may also be likely that the tabs were biased to one side in the same way but not enough to cause a problem until just the right shift in humidity made them all do the same thing.
It’s all speculation on my part, of course, but that’s just how I can imagine it happening.
Best,
Luke
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