Even if people like to think about themselves as rational individuals which take decisions based on logic, the truth is the majority of those people take decisions based on their emotions.
Not only on emotion, but also on other people's opinions. If a famous Influencer talks about a project, a lot of people run to buy without looking.
They refuse to buy bitcoin at the bottom or close to it because they are afraid of making a mistake, however when the price begins to go up that fear transforms and instead of being afraid of losing money they are afraid of missing out, the famous FOMO, also since many more people are buying bitcoin at the time they feel safer doing this themselves, this is similar to what we see when an ad on TV says that millions of people have bought a particular product, even if it is a lie when people hear this they assume that since so many people are satisfied with the product then they will be as well and it is safe to buy it.
People often think that if the price has fallen a lot, it will fall some more. That's why they are afraid to buy. As soon as things recover, people decide to start buying because everyone is buying. Many investors don't want to be an independent unit in the market, they prefer to be part of the crowd, to follow everyone.