I believe AA creates the PNR in between the pax info page and the seat selection page (special meal page for long haul). That locks your fare.

If the seat is unable to be confirmed (for example, when someone snagged the seat before you, or weird married segment logic), you get kicked out to the flight selection page with a 'chosen fare no longer available' banner.

After the PNR is created, you have at least 15 mins to make the payment, provided you move your mouse every now and then to stop the page from timing out. If the page times out, or if you click choose another flight, the PNR is canceled. Otherwise it could actually stay live for a while in the background