I seem to have a cork failure disease.

I made 30 gals of extreme elderberry wine ten years ago.
I still have more than a case left.

It was designed for maxed-out alchohol, I got that and
extreme bouquet, and very sweet, with high natural tannins.
Excellent for a sweet!

Storage has been at room temps, mostly horizontal, but
some vertical time, and some temps occasionally over 85F
due to lifestyle irregularities.

I opened a bottle last night.
I chose that bottle cuz the cork was half stained, figured
now or never. The stained part was damp and crumbly,
and almost spongy, it was not a surface stain.

The wine seemed like it had deriorated. It had lost
that extreme bouquet and fruitiness, thus some complexity
as the tannins and earthyness took the front seat.
It had become a good junkyard wine. ...OK in that context,
it seems twice as potent as whiskey on the rocks, my
normal drink.
The color was garnet, but I seem to remember ruby.
If so, that suggests oxidation.

Many of the corks are showing those symptoms. Half?
They seemed fine a year ago, the wine too.
I have no idea why, I've never heard of this before.
The sugar? The hi-alchohol? Contamination?

Is this a common bug? How to prevent?

Thanks!
--Doug