Basically- imagine that within you is an inert predilection to... liking horse girls, idk.
And for the most part this inert idea isn't a general part of your daily life, it's never acted upon, except in some mild occasional passing. Like an intrusive thought, almost.

But one day, you suddenly step into some shitpost thread, where all everyone talks about, and all you see all over the place, is horse girls. Horse girl this. Horse girl that. Gosh, aren't horse girls great?

And this otherwise inert idea is suddenly either teased, or forced to the surface into actualization.

But, the entire opposite could have been true, too, depending on which environment had been controlled to either seed, or suppress the idea of uma musume.