This is due to the heat from the flames almost completely drying out substance above it, rendering it more combustible than a substance that is moist.

Fires in the sea, by contrast, don't move up hill at all. Firstly because they go out because they are in the water, and secondly because the sea is so flat that there aren't any hills. Unless you count waves, but they are not stable enough to have names or Zip codes so from a legal point of view they don't qualify.