What can cause a foundation to move suddenly after heavy rain?

The correct answer is the increase in hydrostatic pressure. When heavy rain occurs, the ground around a home's foundation can become saturated with water. This water can build up in the soil, creating hydrostatic pressure. Hydrostatic pressure is the pressure exerted by fluids at rest, and in this case, it can force water against the foundation walls.

If the soil surrounding the foundation becomes overly saturated, it can lead to a shift or movement of the foundation itself. The excess water may also cause the soil to lose its structural integrity, leading to washout or erosion around footings. These factors combined can result in sudden foundation movement or shifting.

Other options such as decrease in humidity, high winds, and temperature fluctuations do not have a direct and immediate impact on the foundation movement after heavy rains. Decrease in humidity may actually involve drying out the soil, which typically wouldn’t cause sudden movement. High winds can affect homes mainly through structural stress but do not influence foundation stability directly linked to moisture levels. Temperature fluctuations can cause materials to expand or contract but are not primarily responsible for sudden foundation movement after rain. Thus, hydrostatic pressure remains a pivotal concern after heavy rainfall events.