What I believe it was, or at least what I believe I read, was that if they waited until June 1 to cut Dez he wouldn't have counted against the salary cap at all, but since workouts would have started by then, if Dez got hurt in the meantime they would have been stuck with the full salary. If this is accurate, technically the team could have simply held Dez out of workouts until June 1, then released him. Admittedly that would have probably not been viewed very favorably by players or fans.
It seems the timeline may have been to wait until signing another receiver, then try to trade Dez, and when they found no takers they decided to release him.