It's part of the autopilot package, according to the manual:
If Model X is equipped with Driver Assistance components (see About Driver Assistance on page 73), and you have purchased the optional Autopilot Tech Package, the high beam headlights can automatically switch to low beam when there is light detected in front of Model X (for example, from an oncoming vehicle or street lights). To turn this feature on, touch Controls > Auto High Beam.