First, you have my condolences as bottle of Mead gone bad is
truly a personal tregedy.
Second, you have my respect. The idea of an eight year old bottle
of homebrewed Mead in my house is akin to my claim of virginity. ;)
When was the last time you tasted a bottle from this batch?
This reads like a Solvent or Acetic fault. But if that were
the case, you would have experienced it in prior bottles!
It's very possible that you have a one-bottle infection.
Dick