From what I understand this all stems from the way him and GB parted ways. Dude's royaly pissed about something that happened behind closed doors. We might never know, but from what he has said publicly, they treated him like shit.
From what I understand it has mostly to do with the fact they wouldn't just accept him back with open arms late into training camp after his last year with them when they already had the whole system planned out for Rodgers and designed everything including who they drafted that year for that. That was after Favre said he was retired for most of that off-season. Basically it seemed that Favre thought Green Bay should just automatically take him back because he was Brett Favre and they wouldn't do it. I can't really blame Green Bay for that, and most Packer fans seem to support that decision.
As far as this, I think it might all have been a ploy (that the Vikings might very well have known about and gone along with) to get out of part of training camp.
I think I've come to the realization that the very last events, in order, that happen in our universe before it ends are going to be:
-Peace, justice, and goodwill towards people spreads everywhere-
-Duke Nukem Forever gets published-
-Brett Favre finally retires for good-
-Chuck Norris dies-
-Universe ends/end of time-