Obviously, I haven't seen or examined the fixture in question, but I'll go out on a speculative limb.
The fixture is likely old, and although
VitaminH sees no signs of something wrong, and is properly lamping the fixture, it may well have been overlamped in the past.
That said, overlamping can cause deterioration of the connection where rivets hold fixture wires to the pin and shell connections in the Edison lamp base(s). Once the connection has degraded, the lamp base will operate at a higher temperature than a new fixture, owing to the high resistance of the poor connections. This higher base temperature may be the cause of lamp failure.
Any other hypotheses are welcome, other than shorts. Shorts don't reduce lamp life, they avoid commando life.
