I think this was in Electronics Design: The solder joints for the LEDs were a vulnerable point, and they attributed it to insufficient heat sinking. The lifetime was maximized in the bulb up position and minimized when the bulb was down, presumably due to the difference in the convection. The apparent amount of heat sinking varied from brand to brand.

You can test this hypothesis by opening the dead bulb (voiding your warranty) and applying power to each of the LEDs individually. I think you need 10-12V for each LED.