Recently Bitcoin is regaining market dominance over Ethereum and other altcoins. This is not due to BTC's superiority, but is an incidental effect of the market - specifically the failure of exchanges to offer more "trading pairs". Trading pairs seem to always include BTC and often ETH. Other altcoins sometimes - that's right, sometimes, are offered.
When sellers are either cashing out or seeking safer havens than coins with more volatile price swings, they have few choices and thus exchange their altcoins for BTC. These same BTC owners either hold BTC hoping for the volatility to pass, or they exchange their BTC for fiat currencies (dollars, yen, yuan, rubles, eruos, etc.).
As long as people seek shelter from the price correction in coins, BTC will show heightened levels of trading dominance. When the correction passes, altcoin buyers will leave BTC and again flock to the more volatile altcoins in hopes of catching the larger upward price springs that volatility bring.
Thus, I believe that BTC's increasing dominance is only temporary.