Did the tube fail from vibration? Life? Bad parts? Bad assembly? No QA or QC? I don't know and I don't care. It is not worth the time to find out. If we did know would the maker even care?
This single tube affirmed that you cannot count on current production tubes. Players always need to have a spare set of tubes when they gig or travel AND just because they are new in the box it does not mean they are good. Run them in your amp for at least ten hours before they are to be considered known good spares (even if they will probably be short lived).
This single tube had the potential to teach me something down the road. Instead it affirmed some of my other beliefs.