Market makers. They have to determine a price, but ETH hasn't yet evolved to a point where it operates in its own world, so they lazily (but understandably) fol

Market makers. They have to determine a price, but ETH hasn't yet evolved to a point where it operates in its own world, so they lazily (but understandably) follow the price fluctuations of BTC while allowing people to buy or sell within a certain range. Obviously if a huge buy comes in, or a lot of people are buying, they'll let it go up (or down if its the opposite), but once that dies down they'll again follow BTC price swings. However, I expect this to change as cryptocurrencies mature.