I was talking to an old colleague of mine that was an electronics engineer, and apparently, the issue with the xbox 360 was that the heat inside the case made the solder a bit more softer than expected. This would not be an issue for a horizontally positioned xbox 360, but when placed on it's side (which a lot of people did), this caused extremely slow shifting of certain components on the motherboard towards the direction of gravity. After (for most people) a few years, certain components would shift enough to not have proper contact with the board.
I was offered the analogy of the long-standing pitch viscocity experiment - everything is technically a liquid, just to wildly different degrees.
Not sure if this is true though, it was many years ago!