Source: (https://coinmarketcap.com/charts/#dominance-percentage)
Any ideas about why this is happening?
Bitcoin's dominance is a function of every other coin's collective market cap, so primarily what was causing the fall was the surge in ETH. When ETH was surging, it was more profitable to hop on that train for awhile, and new money coming in to the market was choosing that over BTC in the hopes of fast cash. The trend seems to be reversing now, as ETH is losing ground and new money is once again choosing BTC over the rest of the alts. This type of action is momentum-based and a sort of self-fulfilling prophecy. Everyone wants to own the fastest growing coin, and by chasing momentum, they help to create and maintain it. I would expect the pendulum to start to swing back at a point, unless the altcoins were just a fad that are on the way out. My opinion is everyone trading crypto is too addicted to this form of gambling, and they won't let altcoins die. Too much money to be made trading them, which is why I expect to see BTC dominance start to wane again at some point rather than increase or maintain its present level indefinitely.