The goddess war erupted out of an imbalance. There had always been
three, and the sudden reduction in their ranks left the goddesses at a
loss. The damage inflicted by "Habuki" upon Washu would cause an
unexpected merger between human and goddess, between victim and slayer.
And the two remaining Choushin determined that nothing could be done to
reverse the process. It would take a while, but eventually Habuki and
Washu would blend into something new. The new goddess would have the
original's power, but the consciousnessness of her human victor.

The Choushin are not human. They exist outside of human cares, of death
and birth and the dismal meaninglessness of that which comes between.
These are not of their concern, because they create those phenomenon.
They were never born in the conventional sense, they do not die, and
the dreary burden of normal life does not weigh upon them in any way.
But such intangibility comes with a price: they do not have free will.
They serve the necessity of the universe without question or
understanding, cogs in an unchanging ritual where one goddess hands off
to the next sister at a prearranged moment. It had been a predictable
existence.

Human beings do not normally have the luxury of transcending this
endless round. But they gain free will as compensation, get to choose
their individual destinies. But until Habuki's actions against Washu,
no human had ever amounted to so much.  But she had found an unexpected
flaw in their armor. She had exposed the true secret of Washu's
existence, and the goddess had lost her power to her slayer. It had by
no means been the prearranged fate of Habuki to become a goddess; she
had willed this destiny upon herself.  And that the goddesses could not
understand. Habuki was as a whole unremarkable, it was only be putting
herself into extreme personal duress that her soul became imbalanced
and she somehow generated the power that drove her, and the goddesses
focused on these warring sides.

And they looked at it from different angles. Tokimi was enamored of the
darkness that had given Habuki her power, seeing it as a great
enhancement to their existence. Tsunami considered the light side to be
more significant. This division led to disagreement. Tokimi reasoned
that the universe could not proceed ahead without Washu, and was
willing to accept the new goddess with her superior wild power without
question. Tsunami considered the new Washu too tainted and confused to
fit in harmoniously. They were in an uncomfortable position of being
both right and wrong at the same time.  Divided, they faltered,
consensus broke down, and they began to clash. The tension of one human
soul had been injected into their circle, and they were not able to
adapt to or conceptualize the problem. It soon became conflict, though
again by necessity than by choice.

Perceiving that Tokimi would force the issue and facilitate the union,
Tsunami acted in a decisive and drastic manner. She secured the last
sun of Washu in a great ship of her devising, and returned in it back
to the Earth realm. She returned to an empire doomed by the damage of
the ten suns and the resulting controversy in Heaven. She had only
intended to return for Habuki, for she would use her power to imprison
her former protege within the very primordial fabric of the universe,
to lock her away in darkness from Washu. But the day of ten suns had
somehow affected Masaki as well, for twins are themselves Yin and Yang
and are similiarly interlocked, and the enhancement in power that one
sister received had changed the other by compensation. But Masaki was
more naturally gifted than Habuki, and in Tsunami's eyes she was
something even more profound. With Habuki she sealed away into darkness
forever, and threw away the key. But Masaki would become central to her
plans for redemption. "And the Water God spied the Moon Goddess, and
fell in love with her beauty, and covered and captured in a great mist.
And that is how the Moon became trapped in the Water."

(And there is the possibility that their mother was sealed up as well.)