The reason for this is quite simple.
Currently people are trading their altcoins for Bitcoin to join the speeding bitcoin train and make some profit. However the train will stop eventually. After Bitcoin stops growing so hard and becomes stable or drops a little in price the people will trade bitcoins again for altcoins.
I don't even have a concrete idea about this, but I think one reason is that there's a lot of altcoin nowadays and since there's a lot of choice where to invest to, people are rushing whenever a new potential coin appears so, they would trade their old altcoin to invest to a new one. That would affect the price of other altcoin. I'm not quite sure, but I think it some how makes sense right?