Yeul is the only person who is truly
reincarnated completely by Etro with the same heart over and over (or in the western release "soul") out of the chaos. She technically was the
first person ever fashioned from the blood of Etro and given a heart, and when she died, Etro took pity on her loneliness and gave her the "gift" to be reborn over and over so she could never be lonely and thanks to this, be with Caius forever.
This is a serious problem however.
Yeul's heart constantly splinters with each death and creates innumerable, distinct copies of this same woman who lived. They all retain their sense of self and unique characteristics in life because the same heart comes back to the Unseen Chaos over and over, creating
more Yuel copies that
do not diffuse and instead remain distinct, and individual from the whole.
Imagine if Sephiroth was able to send his core spirit energy to be reborn back to the planet over and over, and every time it died it's new memories, experiences and everything else came back to the planet, stayed distinct from the previous copy, and the cycle repeated over and over as long as life existed on the planet. And all the Sephiroths stayed put in the lifestream, continuously aware and cognizant.
Eventually you'd have a surplus of spirit Sephiroths who's wills are unable to diffuse into the collective flow of souls, and theyare able to tilt the balance of spirit energy in the cycle. Like thick, stagnant fluid that's in a pool but settled at the bottom, which threatens to displace the cleaner water by forcing it out of the limited sized container.
That's what Yeul ended up doing as a collective force.
Etro, (who's really a very shortsighted and clumsy goddess tbh
) created the problem of having difficulty in holding back the Unseen Chaos because too many Yeul's were created by her constantly living short-lived lives and remaining distinct from the flow of chaos. Because these numerous "dead" Yeuls in the chaos remain distinct and aware, they are able to control the chaos as its embodiment and
some have turned dark and even held a grudge. When Etro proceeded to intervene and free Lightning and her friends from their Crystal Stasis, some of the Yeuls as chaos dragged Lightning into Valhalla and from there proceeded to fuck up her fate and that's where she ends up having her adventure and shit from XIII-2 on.
What's not fully explained until Lightning Returns and in the subsequent novella of XIII-2, is that while most of the Yeuls that exist are pure-hearted and kind, there exists Yeuls who are bitter, vindictive, sorrowful and full of spite. These are the Yeuls who curse their fate and wish to have Caius succeed in his goal. The "shadow" Yeul is the one that taunted Lightning and said Serah would join them, kept Snow in place at the Colosseum unable to help Serah in the final battle, and manifested the voice telling Serah and Noel to go back and not to interfere at the Academia in 500AF. She's the one that sends the two boss monsters after them too.
And she is the Yeul who wants to keep Caius in the chaos forever because they refuse to let him go.
In short, because Yeul kept being reborn with the same heart, the heart would keep getting copied, and it would never diffuse into the chaos. Because Yeul would never get diffused and would be copied, those copies just exist in the chaos, multiplying with each short lifespan she's born with, and thanks to never diffusing, they were able to embody and manipulate the chaos. This creates an unstable situation and allowed for the chaos to spill out, fuck up reality, and in the end, undoing the universal balance of chaos.