Vei left because of contract disputes. She just didn't re-sign when her contract expired, purely a business decision, she said so.
We don't know why Nyanners left but it's assumed to be for the same reason.
We don't know why Silvervale left either, it's just assumed to be over personal disputes with other Vshojo girls based on how snippy she got over questions about them in the leaving stream & how shady Froot was acting at the time. But that could just be bad timing and maybe she didn't like the new Vshojo contract either. I don't think there's anything pointing towards problems with management that isn't just speculation - they didn't make any public statements when she was being harassed, but nor did they make any statements about her returning and going on to play the HP game again.
My understanding of the manager relationship in Vshojo is that is works on an "if you ask" basis - talents are generally independent and responsible for themselves until they ask for help on any given issue, at which point management offers advice and technical support or whatever (and is when Vshojo starts taking a cut). Worth noting that I've never really heard anyone say anything bad about Gunrun - he's been a figure in the background of the streaming industry for years and is by all accounts a genuinely good guy really interested in the tech and business behind it.