i don't know the exact reasons why if bitcoin price fall then altcoins also will following bitcoin but in my opinion this is happened with many times because when the price fall then people also will dumped their altcoins too following bitcoin that's why altcoins will always following bitcoin price movement
you know what, sometimes you don't even need to know the reason for it! you just have to know that when bitcoin goes down, altcoins get dumped right afterwards. this can create a lot of good trading strategies for you. for example you place a watcher on bitcoin price and as soon as you see the drop (within seconds) you short altcoins and wait for them to reach the bottom and have a dead cat bounce to make another quick profit.