It is known that the presence of magnetic clouds (MCs) in the vicinity of Earth causes the deepest decreases in galactic cosmic ray (GCRs) flux at ground level (i.e., Forbush decreases). We present here a selection of Forbush decreases, which are associated to non-interacting MCs, and perform a statistical study on the shock/ejecta components for a subset of this events. By looking for correlation between the most relevant parameters of MCs and GCRs, we propose causality mechanisms to explain the observed correlations. In particular, we determine the relevance of each of the physical processes involved in the GCRs modulation, such as: diffusion, magnetic cloud expansion and trapping times of confinement.