Final answer:
Earthquakes, heavy rainfall, and human activity can cause a slope to become unstable and fail.
Step-by-step explanation:
Several phenomena can cause a slope to become unstable and fail, including earthquakes, heavy rainfall, and human activity.
Earthquakes: Earthquakes create stresses that weaken slopes, leading to failure.
Heavy rainfall: Saturation of rock and soil slopes due to snowmelt or heavy rain can weaken them and cause failure.
Human activity: Excess weight from accumulation of rain or snow, stockpiling of rock or ore, or man-made structures can stress weak slopes and lead to failure.