Well, I mean, this late in the game, you've got things like union considerations to factor in, and the project is going to smell fishy to whomever takes over, which tells me that there must have been something really, really big to trigger this.
I'm reminded of Final Cut, Steven Bach's book on the making of Heaven's Gate. There's a point where the film's deep into the slog of production and United Artists is seriously considering firing Cimino. Bach, who was the UA executive in charge of the production talks with another director about possibly taking over for the final month or so of the shoot. The director's never named, but Bach very heavily implies and suggests that it was David Lean. Anyway, Lean's response -- which was phrased very diplomatically, by the way -- essentially boiled down to, "Oh, hell, no, I'm not stepping in on this, and good luck finding anyone who will."
So, either this shoot is somehow going worse than Heaven's Gate (lawl), or there's a huge issue that popped up out of nowhere and thus far no one is talking about it.