Could the reason be that he wanted to see if Wonderboy could take out Woodley for him
Think about it, Woodley is GSP's nightmare matchup. Not only can he defend the takedown, but he will also KO GSP with the quickness. GSP has nowhere to hide if he happen to fight Woodley. There has been no opponent so that gave GSP as much trouble as Hendricks, and Woodley is basically the faster, more dangerous version.
On the other hand, Wonderboy would be a pretty decent matchup. GSP knows that he can take down Wonderboy.
Think about it, Woodley is GSP's nightmare matchup. Not only can he defend the takedown, but he will also KO GSP with the quickness. GSP has nowhere to hide if he happen to fight Woodley. There has been no opponent so that gave GSP as much trouble as Hendricks, and Woodley is basically the faster, more dangerous version.
On the other hand, Wonderboy would be a pretty decent matchup. GSP knows that he can take down Wonderboy.