Have some doubt on it,
When Bitcoin rises, other coins fall because people are selling their alt coins to go into Bitcoin?
Yes that's what it is happening in most cases, people sells their alts to ride the price pump of btc and right now I believe same thing is happening, btc dominance is still high it will be difficult for alts to move, the spotlight is on btc at the moment, institutional investors are pumping in, top companies are coming inboard, maybe alts will be next year.
For what is happening so far is the same thing happened for the past 3 years ago. Only their difference was in terms of price Bitcoin right now surpassed 20k plus in USD$. But it was already started now that altcoins season will going to be happen in the near future, but for now most or some of the altcoins are dumped their price while bitcoin was kicked its value in the market.