Because someone with a lot of money is trading all alts at the same time with a bot, and figured out they can make more money if they crash the market by withdrawing high, and then buying back in really low. 

The reality is, crypto is a tiny market compared to other finance, which means that when big money comes in, it moves everything. When it moves money out, it moves everything.

In the past altcoins were only bought with bitcoin. Nobody who didn't already have bitcoin was buying any altcoins. Obviously this would have the inverse relationship people are suggesting. These days, people are buying into a lot of the major projects with cash through bitcoin and ethereum.
Over time cryptos will have their own ecosystems connected to different real world applications, and they won't be moving in such lockstep. 

As others have said all the crypto is in the same pool of water, and ~80% of the fiat gateway traffic is through bitcoin and ethereum.  https://coinmarketcap.com/currencies/volume/24-hour/#USD