Psychology...

If not much volume occurs on the 1st day of the Bearish Engulfing formation compared to the 2nd day, then this increases the strength of the pattern. The 2nd day opens above the close of the 1st day, however quickly sells off to finally close below the open of the 1st day. This damages the spirits of the longs and brings into question the bull trend which prompts additional selling in the coming days.