You might be using inferior lamps in this fitting - ones with low-hour life.
You might be switching them on and off more often than other lamps in your house - the repeated thermal shock of warming up and cooling down more often might have an impact on their life span.
There might be a poor connection in the circuit that supplies them - the switch or any junction box that feeds them - or a poor connection in the neutral return path. This will possibly have a similar effect as switching them on and off rapidly.
Vibration from passing traffic or loud music could have an impact.
I'm just guessing...
February 2015