I just thought of a possible genius play by Sylvanas and I need to know if that would actually make some kind of sense. Now that you've seen the Proudmoore Reunion cinematic, I wanna say that I do believe it's a good thing that Baine got arrested, he's gonna be protected in an horde hub from any alliance leader who would wanna come and kill the him, believing that he knew Derek would be under Sylvanas's control (cause he obviously is) and just delivered the pawn. Maybe it's somewhat to "force protect" his commander that Sylvanas did this, to protect him and just to draw the alliance to attack the horde and strike back so that she can use the power of Xalatath against them to do a complete butchery and raise them afterwards .
But again, it's just a theory, could that actually be it though ?