Have a look at the preferences page. There's a line there that sees to that your team joins any free PL automatically.
I have to say it's a pretty strange option imo and I will see what I can do to get rid of it.
All I see is "Make your team available for any private league (Supporters Only)" Reading that it sounds like it would make you available for any PL, but not automatically accept any invite that comes your way...That said, I haven't changed anything in my preferences...Typically when a PL season ends, I have several invites to join other PLs...I either reject those invites or join the one I want to join if they have already sent me an invite, and then it cancels out the other invites.
In my near 10 years of playing this game, I have never been auto accepted into a PL, and like I said, I never changed anything in Preferences relating to PL's anyway. Since that PL does not start til Friday, if at all possible I would like to be removed from it.