I guess you use a PMDG airplane, right ?
If yes, that's the problem: it's a trick used by that airplane, to achieve some sort of dynamic lights, before those were possible so, the airplane flies together with a big plane that contains pre-rendered lightmaps for the positional lights, because in simulators before P3D V4, those could not light the "real" ground.
With P3D4, they won't need such tricks anymore, which can easily cause conflicts with sceneries.