Have some doubt on it,
When Bitcoin rises, other coins fall because people are selling their alt coins to go into Bitcoin?
Some times it happens but the situation can be different and it's not always true if people were selling their altcoins for bitcoin. This might happen when the price of bitcoin has dumped so hard.
People will be feeling panic when bitcoin gets dumped and otherwise they will try buy more altcoins when bitcoin has pumped.
Some of the prices are influenced by trends to be honest here. so when bitcoin has the momentum to go up, altcoins will react. of course by showing price movements that are almost similar to bitcoin but not significant