Having been through it in San Diego years back, it is the wind. When it blows hard it doesn't shift direction much. So when it picks up embers it blows them down wind straight.

The wind then intensifies when the 'heat wall' comes through which is when the air is hottest and carries the heat of what is burning nearby and upwind. The heat wall doesn't travel as fast as the wind - it travels as fast as things catch fire.

The result is the fire can have a narrow, straight path as wide as its source. Houses on one side of the street burn and on the other they don't. It is a bit surreal.