From what I know, all the Wagner mercs and other volunteer groups were said to sign a contract with Russian MoD, so there was a plan to nationalize them. Prigozhin, of course, didn’t want to lose his PMC (and sweet sweet money), so he made a strange fake video (and it was so stupid that I and many other people really thought it was some kind of distraction from something) about russian army bombing one of the Wagner camps, and used this as a justification for his coup attempt.