Attack on Titan: This Isn't the Story I Know

After transmigrating, Bai Yu settles down in the Shiganshina District. Although he constantly hones his combat skills through his system, he has little desire to fight Titans. Knowing how the story unfolds, he just wants to keep his head down and let Eren and the others handle things when the time comes. After all, he hates dealing with trouble.

But...

Several years later.

Equipped with omni-directional mobility gear and gripping a long blade, Bai Yu stands atop a wall dozens of meters high. Gazing out at a world still teeming with Titans, he falls deep into thought.

Why...

Why did the entire plot unfold just as it should, only for him to discover, once he left the island...

Aside from the Eldians on this island, every person in the world had turned into a Titan while Annie and the others were on the island!

What... exactly happened?