This is an interesting move. Clearly the Titans are worried about linebacker depth with the injury to Moises Fokou. Zach Diles has been with the team before and was an adequate player. He should be able to come in and help on special teams immediately.
Micheal Preston won the hearts of fans during training camp and the preseason. He has a lot of potential, but he hasn't been able to get on the field to this point. Ruston Webster must have thought he had seen enough to ahead and move on from him.
If he clears waivers, the Titans will probably try to add Preston to the practice squad. I doubt he would have cleared waivers before the season started. It will be interesting to see if he does now.