I think it has a lot to do with the “ladder” mentality. If someone never wore a hat, then not wearing a hat is no big deal. But if someone did, and now doesn’t, it’s important to find out why. Not because wearing a hat (or not) is the definition of OTD, but because it shows a slip in observance, something that should be discussed with him.
(BTW I have a friend who no longer wears a hat who is still perfectly Frum. His family has no problem with him and neither does our very-black-hat community.)