If bitcoin rises, people sell. If butcoin goes down, people sell. Too much btc fluctuations make altcoins trading very difficult.
I do not know why many people sell fall, probably due to lack of experience. After all, in fact, if you allow everyone to the market in a row, completely unpredictable chaos will begin. And in addition, people can become smarter, gather in groups and purposefully destroy tokens. Buying them up to 100% and selling sharply.