Most often it's a voluntary report. Someone feeling sour and decides to report it.
But if a game makes enough of a splash (PE I think was rather high in the ratings), Patreon will go out of their way to inspect the game.
Things seem to point more towards someone reporting the game.
In any case, once that happens, they have to have someone play the game (thoroughly) and report on what content breaks Patreon's guidelines. To which the devs have to change it, have it screened once again to make sure offending content is removed, or gtfo. They're also not allowed to do anything that "breaches" the guidelines outside of Patreon, or risk their page getting shut down. Which is why even though PE has a SS account, they've still got to release the Patreon compliant version there, since they still have a Patreon up.
So for most games of this type, it's less of a question of if and more of a question of when once they get big enough. Most devs seem to skate the line and deal with that outcome when it appears, rather than compromise their vision for something that might not happen if their game doesn't get big enough. Of course, all it takes is a couple of people to report the game.
