As always, it's more helpful if you identify the specific routing (which you did), date, and time or flight numbers.

Speaking generically about travel and not specifically about AA, the most common reason is that the product was never avaialble in the first place, It could be a mistake price, or the pricing algorrithm checked price without checking availability, or something else. I found with regard to a particular car rental that the site detected a duplicate reservation but didn't tell me that was the issue. (I preferred not to cancel the existing reservation before making the new one for fear of a situation where the newly booked price would turn out to be unavailable.)