engine coil falure on 2006 BMW 330i

what would cause an engine coil to fail at 47,000 miles and if all six coils are replaced what would be the expected life span for them.

Asked by for the 2006 BMW 330i
I find that BMW coils go out in short order. I used to work in a BMW shop and we would just replace them at 60k, if they even made it that far. I find that most BMW vehicles are awesome to drive but typically take much more $ to keep them in top shape, kind of like owing a race horse.