From what I've understand so far it looks like mCe and the c9 Val GM/org had infighting issues regarding the main lineup. When c9 picked up zellsis and yay back last year (along with mCe), they wanted to form a super team of the best players they could get their hands on in each category. mCe probably didn't have an opinion on who to sign, or at least didn't know how the team was going to perform. Their mediocre performance paved way for the very controversial dropping of yay and vanity very soon after, replaced with essentially no-namers in the valorant community back then. C9 let mCe build his own roster (similar to what FNS is planning to do right now) from the ground up. But after failing to make it through NA LCQ, c9 probably wanted to take back the reigns of roster-building from mCe, especially now during roster revamp season.
Now the very recent dropping of mCe and re-signing of vanity evidently highlights the discord between the two parties. It's obvious c9 wants to sign more proven and valuable players on the market while mCe wants to experiment with less proven players of HIS choice. The differing philosophies ultimately end in mCe getting dropped likely due to roster arguments between him and c9 management. I may be going off on a limb, but reading mCe's tweet of his unexpected dropping (when a few days ago most sources expected a rebuild around jakee and mce), it is clear he did not leave c9 on the best of terms. Most players leaving an organization would have mentioned a word of appreciation for the roster and org moving forward; mCe simply states that "unfortunately c9 has not moved forward with him" almost in a condescending tone. The signing of vanity almost adds fuel to the flame as it seems like a hilariously timed final act of spite by signing the very player mCe dropped from the team months earlier.
Discord