ETHBTC is up 40% this month. The chart is totally chaotic. ETHUSD (sometimes) goes up after Bitcoin does because people hear about ETH after they hear about bitcoin - most price movements in either pair are due to new money. The prices of all cryptocurrencies also respond together to any regulatory threat.

The weak correlation between ETHUSD and BTCUSD has many obvious explanations.

Most things crypto are suspicious. I don't think there is anything suspicious about the ETHBTC chart.

Edit - This may be informative:

https://en.wikipedia.org/wiki/Beta_(finance)

Arbitrage.

Many exchanges deal only in crypto currencies, where exchange rates are relative to other coins. Price differences between both coins and exchanges create an ability for someone to make money while equalizing the market.

It also means that all coins are inevitably linked together in some way. And why obvious scam coins also follow same curves.

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.