My reading of Bryan's book led me to surmise that he was as dumbfounded as the rest of us. But be that as it may, Mack eliminated certain possibilities, among them the insanity of the witnesses and delusions (unless you consider a mass delusion wherein all the many unrelated witnesses essentially have the same very unique abduction delusion).
Mack’s use of hypnosis invalidates his witnesses testimony irretrievably in my view.