drugs is a common speculation, as are taxes, and neither are theories as they're both based on generalizations and no actual evidence
anyway>>1344068>She didn't seem like a drug addict to me,
I don't do drugs personally but I've met enough people who do to stop assuming I know what "being a drug addict/user" is like. Unless you're doing something hard like heroin or meth it's probably not even noticeable. That said in Japan even weed can and will get you jailed, and subsequently disappeared. It's a big deal. The reason drugs are a common speculation is that being disappeared over drugs has happened a LOT and it would be reasonable to be like, "I found out you smoke weed and the last thing I want is that ruining VOMS' image"
That said yes it could just be a breach of contract because she was moonlighting. Nobody knows, and don't believe anyone who says this or that is "the most common theory" because there are no theories: nobody but VOMS/Gyari knows, of course, and it will remain that way