Advice? One of my PhD supervisors is insisting on involvement with my article based off my Masters thesis.
The standard answer to all questions from graduate students applies here as well: Talk to your adviser. Have a frank conversation in which you bring up your concerns, have him explain to you why he wants to help, and then -- between reasonable adults -- come to a conclusion of the issue. 

You can speculate all day long about his reasons and maybe he about yours. But grown-ups can have a professional conversation that resolves these issues to everyone's satisfaction.