That is because all of the altcoins have pairings with bitcoin so whatever happens to bitcoin happens to all of the coins that are in the market, if there is not enough trade volume on bitcoin then that means that there is not enough trade volume on the coins themselves so as the price of bitcoin goes down, so do the prices of the altcoins go down.
Yes, generally speaking, bitcoin is most recognized by everyone. It can serve as a vane for the market. When bitcoin goes up or down, other coins react with each other.