GSX asks to Simconnect if the airplane is flying or not, by querying the official variable that indicates if the sim is on ground so, either the scenery has a problem with altitudes, or the airplane does something strange with the flight model, which affects that variable.
However, since nobody else reported this with the NGX, it's more likely a scenery problem, but you don't say if you are on an addon scenery or the default one so, we can't really say.
