You mean the ends of the plug wires on the dizzy cap?

That was my thought too, everything was nice and snug, wires/cap, rotor screw, etc. That was the first thing I made sure of when I took the cap back off.

I cleaned the various surfaces with a brass(I think) wire brush. Nothing too abrasive, pretty soft bristles by wire brush standards.
I might as well have used a paper towel on the rust-colored crap 'cause it came right off.
There was also some white build-up on the inside of the cap that I didn't notice the first time so now that's gone but didn't help things.

I'm guessing that cleaning things speed up the process and something got more charge than it had been and poof.