actually what you have been following has been the "altcoin market cap bloat" not bitcoin dominance otherwise you would have known that bitcoin dominance is more than 95%. but when you look at the bloat of the total market cap that is caused by altcoins and decide to call that "dominance" then that is a different story.
and it has been decreasing because a lot of altcoins are reaching their end of line and dying. so as they get dumped and disappaer their market cap disappears with them and the bloat shrinks hence increasing that percentage you were looking at.