When BTC starts dropping, people usually exit alts and try to buy BTC at lower prices, thus causing a cascade effect
Wow! This is very interesting and logical. I believe this. If not the only reason, it is surely one of the reasons. But clarity is still needed here. If people exit altcoins when BTC is dip to get BTC, making altcoins dip too. Why do altcoins rise when BTC start rising again. I think the exited altcoins in certain time is supposed to remain in dip and not follow BTC immediately.