1) having a financial constrain that is bigger than his reserved fund.
2) not having a proper investment plan.
3) having a contract that will give you X2 or x3 profit of your investment in bitcoin, so pulling it out and sell to do the contract, after making the profit of 2 to x3 then you might decide to use 2 portion of it to invest in bitcoin and use 1 for other purposes or you may invest all to bitcoin.
4) not being able to withstand the amount of dip, just like some people may have bought Bitcoin some weeks ago maybe at the price of $60k and btc has risen to $75k and dip down to $63 to $62 if it forther dip below the $62k down to $55k some investors will panic and sell to watch how dip it may fall before they can buy again because most people may not allow their btc holdings to dip below there expectations which they have set.