This isn't any special correlation between Bitcoin and Ethereum, there is a certain correlation between the whole cryptomarket.
Primarily other cryptos follow Bitcoin in it's general price direction, but not the other way around.
This is simply because price is not determined by technology in the short term, but by opinions of average people who know nothing about cryptocurrencies.
Everyone knows Bitcoin is the main cryptocurrency here, so when it falls, people get out of the whole cryptospace, when it rises, people look into other cryptocurrencies as well. When you see price movement of some altcoin, you know that it is just small part of the whole market, so you don't react the same for the other cryptocurrencies.
You have to see this from the perspectives of someone who doesn't know anything about cryptocurrencies. They control the price.
If price was set by people who knew about how cryptocurrencies work, the value would only increase at an exponential rate.