I've wondered about sinus issues, but it seemed weird that it would only bother me when I'm tired. A bit more reading later, that seems less unlikely. Maybe I'll explore nasal irrigation and see whether that makes a difference (I've been meaning to for a while).

I totally agree that the alcohol thing doesn't seem like a metabolic issue at all. It's a very immediate reaction, usually when it's still in my throat. Your guess sounds as better than most I've heard, although I doubt I'll be getting a definitive answer anytime soon.