[3rd_gear_Ted]

In the reliability engineering world an event like this is known as a Black Swan event.
The confluence of multiple factors to create a catastrophic event like what happened.

On another note; For a while now swimming pools have been frowned upon as wasteful opulence in SoCal, I think that after this event a 20K gallon hillside lot swimming pool will now be considered a valuable piece of infrastructure that can be used to protect the home. The anecdotal stories of the firemen pumping pools into their fire trucks to carry on the fight are out there along with homeowners who used their pools to fight the fire and save their home.