Nate is not a slot receiver, it looks as though they are strictly looking for a reliable underneath guy for Locker. I'm assuming they want to line up Kendell Wright out wide. So Nate may not be a scheme fit anymore, plus that 4+ million salary isn't doing him any favors. They are finally trying to establish an identity/system on the offensive side of the ball, and Nate looks to be the odd man out.
Indications are that nate "mailed it in" and gave a lackluster effort during the latter part of last season,while being counted on and paid as a leader.It obviously has angered some within the Titan organization.
Source: Washington prefers release