The reason of soil flocculation is

Flocculation refers to a state when primary soil particles (sand, silt and clay) are drawn to each other by inter-particle forces to create microscopic floccules or clumps.
The reason for soil flocculation is the Lowering of Zeta potential. Zeta potential is a property of charged interfaces within the soil solution. Thus, when it is lowered, the soil particles clump and floccules are formed.