The shape, or "slope" of land has a strong effect on the speed a bushfire will travel. Bushfire can spread significantly faster than normal on an uphill slope, and similarly, will decrease its speed on a downhill slope.
A fire will burn faster uphill because the flames can reach more unburnt fuel (e.g. trees) in front of the fire. The heat radiating from the fire pre-heats fuel on the slope ahead of the fire, causing the fuel to start burning more quickly. By increasing in speed the fire also increases in intensity, becoming even hotter. As a general rule, the fire will travel up a 20 degree hill 4 times faster than it does on flat ground.
The opposite applies to a fire traveling downhill: because there is less directly in front or above the fire, the flames reach less fuel, meaning there is less radiant heat to pre-heat the fuel ahead of the fire, resulting in the fire traveling slower.