I'm thinking this might have something to do with Shion rather than Aqua, Aqua is just the one who decided to deliver the bad news since they're good friends.
She hasn't said anything about bad health or anything, as far as I know, she doesn't have a financial reason to leave Hololive, and Friend C has also retweeted this and she was good friends with Shion.
So yeah, my best guess is something bad happened to Shion.