What causes spark plug terminal to corrode through inside insulator?

[Robert_P]

Encountered something new. I finally got around to changing the plugs that were in my '99 Explorer when I got it and on one of the plugs when I pulled the boot the plug terminal came with it. Just pulled right out of the center of the ceramic insulator. As it happened when I put a wrench on the plug to unscrew it, the insulator snapped off which gave me a good look at the stub of the terminal. Both the end of the part that pulled out and the remainder in the plug were well rusted at the junction where they came apart and there's blue/green residue.

I'm sure the terminal corroded through a while ago - I'm guessing this explains the intermittent missing I've been experiencing.

I'm sure this happens, I've just never encountered it before. What would cause this? That terminal seems to be pretty well protected from moisture since it's buried in the ceramic and tightly covered by a rubber boot. I would think the high heat would chase away moisture too.