Not all cryptocurrencies rise or fall at the same time. Depends on a current trend, the number or rising ones would be higher than the number of falling ones and vice versa.
Let's see the image for Pathways to Alt season. It is taken from
SecretsofcryptoBitcoin is a leader of cryptocurrency market with highest trading volume, oldest history and most dominating role.
A bull run or bear run of crypto market has to be triggered by Bitcoin. That is presented in the infographics as Bitcoin first, Ethereum later, top altcoins and low-capped, shit coins.
In aspect of capital flow, you can explain this flow too. Institutional investors, whales have to pick a coin that has enough high trading volume for them in order to easily enter and exit. Bitcoin is the best choice in this term and the second best one is Ethereum.