Originally Posted by MathMusic

Believe it or not, AA's IT holds a seat and creates a booking record for you before you get to the point where you can view the seat map. It's not obvious but that is what they are doing under the hood.

You might notice these temporary bookings briefly showing up in your AA app if you get to that point again. There is a better way of viewing the seat map I referenced above that doesn't require a partial booking to view.