Mycelium said:
Have a look at this image:
http://i.ehow.com/images/GlobalPhoto/Articles/2143119/Christmas-light-main_Full.jpg
Just above the bead of glass that stabilizes the electrodes/filament
supports you will see a few turns of silver wire wrapped around the
supports. This is the wire that causes the bulb to fail shorted. Under
normal operation only the bulb's nominal operating voltage is present
(2.5V-12V AC depending on the number of bulbs in the string) which is not
enough to burn through the oxide layer. Once the filament breaks the full
line/mains voltage is suddenly put across the bulb and the oxide layer is
penetrated causing the silver wire to short to the supports which makes the
bulb unit itself a short circuit. The result is that the rest of the bulbs
in the string stay on and get slightly brighter because of the slightly
increased voltage across each of them. The raised voltage also shortens the
life of the rest so they end up burning out in a cascade effect until either
A: they all burn out and short which blows the 3 amp fuse in the plug, or B:
the set gets jarred or shaken which knocks loose some of the shorts which
then don't reshort because the line voltage is distributed across all of the
opens and is not high enough across any given one to cause the bulb to
reshort.
Both outcomes result in a dead string of lights so it is best to change a
burned out bulb as quickly as possible to extend the life of the set.