when major indices crash, meaning the market tend to lock profit and hold cash

imagine your were having paper gain 100% with 100k, tomorrow it crash 20%, become 80k, your sentiment will tell you to lock the profit because you dunno how low can it goes.

and this sentiment is a chain effects, when big brothers indices crash, meaning (most of the time) the market sentiment is bearish, everyone will try to lock their profit which cause the market to sell down further\

but of course sentiment is just one of the reason