For me it’s not height so much as high edges. Airplane, no problem. Edge of the garage roof...uh oh. Same thing on narrow mountain roads. And something like that glass-floored thing that sticks out over the Grand Canyon? Fergetaboutit!
Maybe that’s my problem, too. Whenever I’m on a high edge, that’s when I get that pit in my stomach. However, the WTC thing happened before the doors even opened on the elevator. So maybe a bit of both?