Do we know what caused the American Airlines outage?

There isn't enough information here to pin it on DNS or a certificate. Both are possibilities, but neither should be treated as the cause without an incident report or technical evidence.

There was an unverified suggestion that a firewall or routing problem might have affected multiple systems at once. That's a different possibility from DNS, but it's still speculation without confirmation from the airline.

The reported passenger-facing problems included trouble retrieving or printing boarding passes and checking flight records. Those symptoms tell us which services were affected, not which underlying component failed.