I wouldn't go so far as another poster did by saying that all carriers overbook their premium cabins - some do not.

But overbooking is only one of several reasons why you might get bumped from a premium cabin despite holding a confirmed reservation. Don't assume that the reason was overbooking unless you know that for sure. Another situation where this can happen is when there's an equipment substitution. For example, the flight was originally planned to be operated with an aircraft having 50 Business Class seats, but at the last minute it gets swapped out for an aircraft having only 40 Business Class seats. If they sold out the original 50 seats and everyone shows up, there are going to be 10 Business Class passengers that need to be bumped/downgraded even though they didn't oversell the flight.

Also, always consider that another option rather than being downgraded is to agree to wait for the next flight where you can be accommodated in your ticketed class. Assuming this works with your schedule, this is often the better option, especially if you have a bad back or otherwise will be miserable in coach. I've been fortunate enough to never have been downgraded, but if I was on a trans-oceanic flight, I'd almost certainly opt for waiting for another flight (even the next day) rather then doing it in economy, unless there was a hard scheduling issue where I really needed to be somewhere (and if that was the case, I probably would have factored this into my travel plans to start with and built a day of slack into my schedule).