Most likely due to over-leveraged longs waiting for "alt season" there were 2 massive spikes in ETHBTC, which caused a dump in ETHUSD which caused a flash crash and this most likely contributed to the selling of BTCUSD.
Now it seems to have stabilized however there might be more dumps if ETHBTC breaks the 0.02 ratio.
The dominance is rising strongly and the Alt market is tanking to the prices equivalent to what they were during 2016. ETH/BTC were at 0.02 only during March 2017 and when the alt market gained momentum we haven't been seeing any slow downs in the alt season since then. Not just the ETH, but other good coins like XMR and DASH have reached the state as they were during the 2016 year. After August 2016 pump in XMR, I have never seen the prices of XMR/BTC trading reaching a low of 0.007BTC per XMR. Similarly DASH had never been 0.01BTC since July 2016.
Apart from Bitcoin, I am a believer of ETH (more importantly the technology behind it) and Monero. Ethereum and Solidity collaboratively will surely help the businesses to take a new innovative way of using the blockchain and solve major real world problems. Similarly Monero's secure transactions and fungibility help us to be anonymous and gives us full control over the privacy.
an interesting theory about ETH and the alt market effects though, I have talked about it before during certain drops last year that I saw the huge dumps in altcoin market that indicated exit of a lot of money that put a lot of pressure on bitcoin price.