That means that some sensor (either on the DCA Flight itself, or on its AAWC controller) detected the ground BSE. There’s no check that says “are you an airplane” (though there should be), so a vector is assigned to the target. Only when it gets to AirToAirAdjudicator»adjudiateAttacker:defender: does it break, since the land unit does not understand underAttackFromA2a.

Two recommendations.

• First, it could be that there is an extra sensor on a DCA or AAWC platform which detects ground BSEs. Any BSE detected or fed to the AWACS will be treated as a DCA target (there's no “are you an airplane” check).
• Second, a sensor (probably CAPAirFlightC2 or AEWAirFlightC2) might be assigned the wrong BSE type interests (data view » SensorData » BSE Type Interests » [C2] » [Sensor] » Domains (could contain Ground or Environment).