Actually, Bitcoin also has had a big growth from July, but now Altcoin season is going on so altcoins are likely to be more active than Bitcoin. What is more, the bigger market cap is, the more difficult it is to raise the price, so it is harder to pump bitcoin than low market cap altcoins. So altcoins are more volatile than Bitcoin. In case of bear market, people get rid of altcoins first as they are considered to be more risky.