Don't they usually share producers/promoters if they're on the same label? Could have a lot to do with it. Or they're just copying what other's have done because they think it's part of what has made them successful?
^ complete guess, can't be bothered to actually look into it when it's obviously BS. I can see how you can get enjoyment out of reading into it, but I do hope you don't start believing any of it :p