My theory is someone has her trapped in there trying to figure her powers out, maybe even how to extract the Mind Stone energy out of her. She seemed fine during Endgame so the idea of a mental breakdown due to Vision dying doesn't make much sense to me. I also don't think it would be an earned storyline because in the movies their relationship wasn't developed to the point that losing him would affect her more than losing her brother.
I'm going with someone is experimenting on her and unleashes her reality-altering powers in the process.