Supply and demand, when people start selling alts they are in turn increasing the demand of Bitcoin and vice versa
It's not true at all. That depends on what trend that is happening. it's not all of people were always doing it. Imagine when you are holding BNB and then would you sell your BNB for bitcoin while it has the same potential to go to the same as ethereum? i think that people will always keep what they are thinking as promising coin and that's why it's not always happening.