Because they don't really understand the cycle of bitcoin, they feel unsafe buying bitcoin at low price because they think it is likely to drop even lower. But when it rebounds it gives them a sense of security and thinking it will go even higher, they can buy from there and wait to take profits. Unfortunately, things don't go as they planned and they will panic sell when the price drops afterwards.
This is very normal, if everyone is like us, just waiting to buy bitcoin at low price, who will buy at high price and how will we sell if no one buys. Financial markets always have winners and losers, not win-win.