Suddenly Misaligned Headlights Explained
1) A damaged lens is the one of the reasons. This issue happens particularly in the older vehicles with bulbs that has the diffusion discs like H6024. The main function of the diffusion discs is to give even light distribution. If these diffusion discs are struck by a rock or hard object and cause a chip or break, the light distribution would immediately become uneven. The newer projector headlights would not have this problem, but if the projector lens starts to accumulate dusty haze and become opaque, the light distribution will become uneven as well.
2) One obvious reason is that something has broken. The headlights are attached to its housing and the car body, and they are held by a few joints and adjustment screws. Since the housing parts are mostly made with plastic, the housing can become brittle over time and can break even with a small amount of pressure, like vibration while driving or hitting a pothole. If you start to see the lights erratically flap all over the place, the issue might be the broken headlight assembly.
3) Some vehicles with the headlight leveling system can face an issue. In normal operation, the headlight beams are supposed to move up and down when the driver operates a manual dial or control system. With HID Xenon automatic system, the headlight beams move down when extra weight is added to the vehicle. Unfortunately, with anything mechanical, this system can go haywire. A bad contact or just plain malfunction would cause the headlight beams to constantly go up and down or stay down.
4) With the filament-based halogen bulbs, age plays a factor. As the halogen bulbs age, the filaments expand and at a certain point, the bulbs are no longer positioned in the same focal point of light distribution. This is one of the good reasons why you should always use quality brands like Philips, Osram, or IPF. Compare to cheaper, no-brand halogen bulbs on the market, this issue is extremely minimal at best. Having said that, it is always a good idea to replace the older bulbs before they burn out, and it is another reason for us to stress that changing the headlight bulbs in pair is important.