Why? Well, I can only speculate, but my guess is that Overstreet - who had declined a previous offer to return to the show on a recurring basis - realized that "concentrating on my music" actually translates to "disappearing into obscurity" real quick. Let's be honest - who is seriously going to buy a Chord Overstreet album? Glee fan girls, yes. But the general public? Nopers. And no one will buy a second album, because by then there will be plenty of new cute faces on television for little girls to love. Maybe he just realized that with a name like Chord, there's literally no one other than Ryan Murphy who's cheesy enough to hire him. I mean, come on. Your name is Chord Overstreet? That's like one of those games where you try to find out your porn star name and you use your first pet and the street you grew up on - which, by the way, would make my porn star name Goldie Union, if anyone was wondering. Not very sexy.
So anyway, good ol' Sam is coming back. No word on how they will explain his return, or what it means for the characters. When we last saw Sam, he was developing a secret romance with Mercedes (plot continuity, what?) but now his family "moved to another state" and Mercedes is dating Tinker from Friday Night Lights. Maybe he'll cause problems in their relationship? Or maybe he'll hook back up with Quinn? Personally, my fingers are crossed for a very special coming out/date rape awareness episode where he roofies Blaine after the school dance. Just putting it out there.