[Tora Newton Y.]

While I can't recall for certain that there was dialogue confirming Papierwaite was the one that stole the Toybox, I'm pretty sure that's implied to have been what happened. While Sam was away and Max was enthralled by the film, Papierwaite stole the Toybox, which freed Skun-ka'pe, who then took Max's brain, is I think how it went.