It's not that they don't believe in Bitcoin, the reason they sell Bitcoin is because they want to make a profit from the transactions they make, so in this case they use the law of "buy at a low price and sell at a high price". People who sell Bitcoin cannot be judged as having no confidence in the asset, they are simply taking advantage of price fluctuations to make a profit.
Investment strategies are not only long-term, when they have reached the specified time limit and obtained the desired profit, they will immediately sell it and then re-enter when the price is low.