If you only had one do it, and it appears to have been awhile, it more than likely was an anamoly. Maybe a bottle with a weak spot in it, or perhaps an infection in ONLY that bottle. I wouldn't worry about. I find that every once in awhile in batches, that the rest of them are fine.
It probably happened in the first 3 weeks of carbonation, which is usually about how long a beer takes. And if it hasn't happened to any more, I would tend to think that will be the only one.
I'd be careful when I opened them, but I would stress out about it. If it was an over primed batch OR an across the board infection it would have more than likely been a chain reaction.