Probably rain getting in. My garage light does the same thing. Since its on the outside, by the time someone notices the light is out, the rain may be gone. And they may not be on during one rain, so the bulb may not burst every time it rains. So, I'll bet weather is a factor even though it may not seem that way. Drop by after a good rain and look for a puddle of water in the bottom of the broken bulb.
