Have some doubt on it,
When Bitcoin rises, other coins fall because people are selling their alt coins to go into Bitcoin?
Possibly but we don't have any evidence for that well we could just look at the previous scenario when Bitcoin starts to crash on 2017 some people throw their coins to altcoins and the Ethereum got its ATH.
There are lots of evidence for that thing, you can try to see the chart of bitcoin in the past and you should try to create a comparison between the chart of bitcoin compared with the chart of some major coins like ethereum, XRP or litecoin. The rises of bitcoin are giving a huge impact on the price of bitcoin.
I thought that if you have seen ethereum was being pumped due to the bitcoin bullish trend too.