Here's what I found on the NFL network about the penalty.

The call was unsportsmanlike conduct. Penalty called on GB bench for coming onto the field, after the play was ending. Apparently they received 3 warnings from officials.

Watching the replay, I don't know what the hell the ref's expect the Packers to do after a fair catch and the ref's throw flags and signal the play is over. They must expect them to raise their hand and say "may we please come onto the field with our offense now?" Total bullshit call.