For me, it's different now.
It's very rare now to happen that if Bitcoin dumps, altcoins pump.
But before, like around the bull market 2017-2018, that's the common behavior.
But now, if Bitcoin dumps, altcoins dumps too. And sometimes it also depends in Bitcoin dominance.
I think not, because I noticed that there are still alts last time who rise when BTC is down. Solana for example. There is also ETH. I know there are more than them if we always track the crypto market statistics. We also have what we called alt season. It usually comes after the Bitcoin bull run. It's when majority of the altcoins are pumping.
In the year of 2017, both alts and BTC are pumping on the last quarter of the year but in 2018, both of them have dumped. This is the cycle of bear and bull. And yes, it's common in any financial markets. I also agree that the pump of altcoins will depend on the BTC dominance, because if the dominance is less, that may mean that people are going for altcoins.