We don't know anything of the "unresolved moral concerns" that may have been connected with Williams's departure from Trinity, although let's face it, responsible adults (if there in fact are any) among the Bush-ACA faction at St Mary's should have been fully apprised of them, and they should have been given a full opportunity to turn down the deal. (I will say that there are ladies of advancing years among that group who are quite capable of saying "Immorality? Sounds intriguing.")
But after all, this is the ACA. St Mary's has a "vestry" that hires a rector, but not really. It also has an absentee "rector", who theoretically is the one who hires other clergy. I would assume that if he's the "rector", it would have been his decision alone to hire Williams as "pastor", whatever that means. My guess is that Frederick Rivers, in his capacity as "rector", did what Brian Marsh told him to do and hired Williams, who had to get out of Dodge in a hurry, and St Mary's was a convenient place to park him.
The best thing that could have happened that might lead to a rector's dismissal on moral grounds would be an affair with an adult female parishioner. There's no guarantee that this is what happened, though. There's also the thing that used to be quaintly called a "morals offense", which might cause greater disquiet among conservative Anglicans. And then there's ACA Canon 46, "Of Offenses for which Bishops, Presbyters, or Deacons May Be Tried"
Section 46.1 A Bishop, Presbyter, or Deacon of this Church shall be liable to presentment and trial for the following offenses, viz:That happened, after all, to Williams's colleague "Bishop" Hiles, and to the author of the whole freak show, "Archbishop" Falk, when both were Episcopal priests and had gotten a bit frisky with the pew. As these bishops see things, no big deal. We all have our secrets, after all, we'll just keep this little business among ourselves.
- Crime or immorality