It's about giving the guys behind him minutes and letting them develop. Both Keita and Davis have the stuff to be big for the club, plus RBNY has a good track record of developing centerbacks so Redding should pan out after a bit with RB2. Combined with that DP deal the team is trying to do and we need the space in midfield.
The trade is more about other stuff going on around it and not so much the trade itself.
We'll see about Redding's development, I wasn't encouraged by his play in Orlando. I haven't been following the recent NYRB homegrowns but I think they'll really struggle next year to replace Sacha. In the end, I just think the trade is horribly slanted in Orlando's favor, and don't see why the Red Bulls couldn't get more for Kljestan.