Feels like a simple math problem. They have 9 WRs on the current roster, but only 7 of them have any kind of chance of making the final 53: Diggs, Hollins, Douglas, Boutte, Bourne, Polk, and Baker. They're most likely to keep 6 on the opening roster, so at least one has to go, but likely two if they draft a WR as most expect.
Given the guaranteed money involved, Diggs and Hollins are likely locks. Douglas has been the top receiver in yards each of the past 2 years, so he feels close to a lock as well. That means two of Boutte, Bourne, Polk, and Baker likely need to go. I think they're hoping to have the problem partially solved by someone trading for either Boutte or Bourne (or even both), and then maybe Baker is waived with the plan to get him on the practice squad. Polk probably survives another year.