Sam Goldwasser wrote:
While the scenario of the bits of the filament shorting is possible with
some lamps (usually with long thin filaments, though unlikely with the
short filaments of halogen lamps), the more likely cause is the arc
resulting when the filament opens. This results in the arc moving
towards the filament supports, with a lower resistance than the filament
had originally, leading to a high current.

clifto wrote:
What I never figured out is how the arc is maintained for more than 1/120
of a second when there's no air around the arc to ionize.

Tiny bulbs are often evacuated--but larger bulbs are filled with gas.
The gas pressure helps prevent the filament from evaporating
(which would not only thin it prematurely
but would lead to metal deposition on the inside of the glass,
dimming the light output).

Look at the light bulb aisle next time you go out.
Standard bulbs are filled with Argon.
"Super" bulbs are filled with Krypton or Xenon.