He made some news last month when the 6th round pick in last April's draft was released by Oakland before training camp even began.
To make room for Sulak, they placed TE Tory Humphrey on I.R. with a broken arm, which ends his season. This move doesn't surprise me; they need that roster spot and can't hold it open for a 3rd string TE who might be able to play again in several weeks.
So why would the Packers want a guy that not even the Raiders wanted? I don't think Sulak has any chance at making the team, absent an outstanding showing during the preseason games. But the Packers are a little thin at linebacker at the moment. LB Nick Barnett is still being held out as he recovers from knee surgery. Rookie 7th round pick LB Brad Jones has missed all of training camp with a hamstring injury. And rookie 1st round pick LB Clay Matthews has been in and out of practice with his own hamstring injury. Sulak's here to provide depth. And he'll get an outside chance at actually making the final 53 man roster.