I'm just guessing that either or both teams are looking to extend the trade and move other veterans. The core of the deal, including picks and young players, might be agreed upon, but it would make a difference if either team is willing to take a player like Westbrook to dump longer term salary. In that case, the Lakers might take Fournier, Conley and/or Rudy Gay, and another Lakers' pick gets moved.
In other words, there may be an opportunity to tie the Mitchell trade to something else that's being held up. That said, if a team like the Lakers were involved, you'd think something would leak.