The topsoil becomes hydrophobic for a time due to the vaporized oils in plants and a bunch of physics I am fuzzy on. This means they’re less likely to absorb any water during the next rains after a wildfire and you get what are called post-wildfire debris flows (a type of mass wasting/landslide). They’re so predictable that planning for them and predicting where they will occur is a regular part of wildfire response in certain states.

Your boots may sink into soup and add any sort of slope and that hillside is going down.