I see a lot of people ask this question, but the common reason here is more of human factors, remember it's real people that trade cryptocurrency, even when robots trade they are simply made to imitate humans and do exactly what humans should do while trading. So the common thing that happen is when Bitcoin price starts sliding down, people tend to move their coins from BTC  to USDT  and to Altcoins to make room for purchases when it starts recovering upward and thereby making Altcoins to move up, same thing happens when BTC begins to move forward, people sell their Alts to buy BTC and Alts price moves downwards the more