After rainfall, groundwater pressure is built up and this elevates the ground water table. The water inside the pores of soil reduces the effective stress of soils. Since shear strength of soils is represented by the following relations:
Shear strength = cohesion + effective stress x tan0 where 0 is the friction angle of soils
Hence, the presence of water causes a reduction of shear strength of soils and this may lead to landslide. On the other hand, the rainfall creates immediate instability by causing erosion of slop surface and results in shallow slope failure by infiltration. In addition, the rain may penetrate slope surface openings and forms flow paths. As a result, this may weaken the ground.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.