detection of plagiarism or 'academic misconduct' of some sort would be the main reasons for revoking a degree.
Buuut I'd be suspecting he flunked the degree outright and is now B/S ing everyone to explain why he doesn't have a degree...but if he doesn't want to talk about it I'd not push him on it.
AFAIK it's really pretty unusual to revoke a degree that's been awarded - the LSE revoked Gadaffi's son's PhD after quite a large media uproar, but they didn't seem to want to do it and acted like they'd have been quite happy to let him get away with it if they'd been able to keep it low key.