Can someone in the know explain why this is actually happening?

Is there actually russian influence on the organisation that needs to be rooted out or is this an internal faction battle?

I am also curious to know if this is in anyway related to the corruption investigation that was happening on V Zelensky before the invasion. (IIRC)

I don’t know the exact situation, however right now in Bulgaria, the anti-corruption commission is one of the levers used to harass political opponents.

It would not be too surprising if that’s the case.