This morning Beane essentially provided the reasons for the big trades. when discussing Watkins, he mentioned whether a player wanted to re-sign and at what cost; with Darby he mentioned scheme fit; then said Dareus has been discussed ad nauseam. He didn't state these explicitly, rather he was making a general point that trades have different circumstances, then blurted out different possible reasons that just happened to coincide with the particular player mentioned.
I think those coincidental reasons are consistent with what many have argued here. The important factors driving decisions: culture, scheme, and cap management. Darby was a scheme/culture casualty; Sammy and Dareus were culture/cap casualties.