At the risk of stating the obvious, from what I've seen firsthand over the past 20+ years is that the members of organizations and the staff of organizations often live in two different environments and have notably different worldviews. Staffers often feel a greater "allyship" (to use the term) with other organizations than they do with their own membership, having sometimes moved to and from those other organizations in their careers.

A strong and close oversight by an active board of directors that does represent the members' interests can help correct this. However, in some organizations staff have been known to control the board nomination and election process to ensure "smooth and harmonious operation". Until the members weigh in by no longer being members. Two organizations that I have been involved with where the bylaws require the board members to be elected by and represent specific member groups seem to have been more successful in resisting this trend.

I don't have enough information to say if this is the exact sequence of events that happened in ACA. For decades, ACA's physical location thousands of miles from K Street seemed to help insulate it from the pervasive groupthink of other national organizations. But with instant communication, social influence, and other cultural factors, that may no longer have been the case.