It's not just ETH but the entire altcoin market. It's been going down for over two months now.

This means that people are beginning to dump altcoins for Bitcoin as the BTC market proves to be most profitable market today, plus there's a bullish sentiment surrounding Bitcoin so it's not surprising to see a shift from alts to BTC from investors, general public, etc. But we've seen this pattern before, from January to April of this year we saw the same increase in the BTC dominance, and it took only one month to go from 45% to 36%. So I wouldn't say the BTC dominance will stay at 50% for too long.

Once the Altcoin cycle begins and the market starts gaining confidence from investors again, the BTC dominance will go down as more capital will enter the altcoin ecosystem.