It's certainly not just you, since this has been discussed numerous times (most recently, here: http://www.flyertalk.com/forum/ameri...-airlines.html), but it's also certainly not new.

From AA's own Web site:

DOMESTIC TRAVEL
In addition to checking in 30 minutes before departure, you must be present at the departure gate and ready to board at least 15 minutes prior to scheduled departure time to retain your reservation and a seat.

The "flat tire rule" isn't a rule so much as a courtesy sometimes extended but certainly not carved in stone. Since it sounds like your "flat tire" occurred in the AC, I doubt you'd get much sympathy from a gate agent (or most FTers, for that matter).

There's a reason they print a specific boarding time on tickets, y'know.