It’s not some massive personal secret. The TLDR version is essentially he had a cult following and cult personality and they didn’t want that so they split ways. You could also argue that Matt was making UOR media his personal playground. Regardless, They wanted a team and didn’t want a personality bigger than the brand itself, which is where it was headed.
Now Matt is with Disney.
There’s your “secret”. Back to Hagrid’s and all the newsworthy info.