Understanding the regional trends in Indian summer monsoon rainfall has important implications in developing appropriate adaptation strategies. Here, we provide a mechanistic explanation of the observed asymmetry in the trends of seasonal mean rainfall during past three decades over the eastern and western parts of the country. This asymmetry in trends of the seasonal mean rainfall is primarily contributed by a similar asymmetry in the trends of low and medium rainfall (RLMR) events explaining 85% of the seasonal mean. As the RLMR generally comes from mesoscale and synoptic systems, it is likely that the observed asymmetry may arise from some asymmetry in changes in large scale dynamic and thermodynamic parameters. We show that increasing vertically integrated moisture transport (VIMT) over the Arabian Sea (AS) is likely reason for the increasing trend of RLMR in the western parts during recent decades. In contrast, over the eastern parts, the RLMR is decreasing due to a decreasing trend of VIMT over the Bay of Bengal (BoB). The increasing trend of VIMT over AS arises primarily due to increasing trends of low level wind speed and moisture content while the decreasing trend of VIMT over the BoB seems to be due to decreasing trends of low level wind speed and moisture content.