Tracking down issues that happen exactly once, or happen only sporadically less than a handful of times per year can be extremely difficult. It's tough to say precisely what caused each of those independent events, and it's even tougher to say what you can do to prevent future episodes.
Unless it happens regularly, or is a reproducible error, I think trying to find out what caused it is a losing proposition. Wait until it happens again, then try and associate like events common between occurrences. That's about all you can do.
Oh, and Google.
Good luck!