Author: Okay, my main couple’s story has become kind of stale, so what should I do? Maybe progress their relationship a little? Nah — let’s introduce a second ship with more interesting characters, better chemistry, and have them become a couple before the main characters.
Fair assessment. I like to call it the "rough draft effect" where the Author's earliest ideas are just okay, but then he / she thinks up some really good characters later, which are better than the original ideas.
This often happens with love interests, where later love interests are better fleshed out, more interesting, and better looking than the ones the manga launched with, and then instantly lap them all (especially the main girl / guy) without even trying, but then for some reason, the best girl / guy loses anyway, because the editors or whatever decide that the first person that the MC bumps into must always win, even if they're the most boring character in every way.
In this case, it's TWO characters instead of one.