Answer to Question 1
The angle of repose is the steepest stable slope angle for an unconsolidated material. If the angle of a slope is less than the angle of repose, it is unlikely to fail; however, if it becomes steeper, mass wasting will occur and bring the slope back down to a stable angle.
Answer to Question 2
Forest fires remove vegetation by burning. Vegetation binds the soil together with roots and also takes up water through the root systems. Once vegetation is removed, the soil is particularly susceptible to mass wasting, especially induced by water.