RESPONSE
Age is the most common reason for a bulb to fail. No different then the light bulbs in the house. The exact timing of each bulb going out is not predictable.
- Corrosion
- Loose connections
- Poor manufacturing of the bulbs
- Possible exposure to moisture
- Defective wiring harness
The only thing these all share in common is the ground connection. And they would not work at all without it.