They don't always move together. Sometimes they do, which can be a sign that new money is flooding into all areas of the market, or it can just be a consequence of BTC trading pairs - i.e., if a coin is trading at 0.1 BTC, and the BTC dollar price increases, then it is likely that coin will still trade at 0.1 BTC, so effectively its dollar price also increase.
Sometimes BTC goes up and alts go down... which means there is a lot of people buying BTC and a lot of people selling alts - so generally these will be the same people, i.e. they are selling their alts for BTC. This sometimes happens at the start of a bull run, with BTC leading the way, and later on when BTC stops increasing people sell BTC and buy back alts, causing alt prices to rise.