[glassdoctor]

Might be a simple as a bad solder connection that "fixed" itself with the handling of it...

but more likely it's a bad cell (internal short that was blocking voltage) that woke up when you hooked up the charger, thus allowing the pack voltage to flow again.

I've seen old packs act a bit like this.. they have cells that drop voltage really low, but the voltage goes real high when charged... I can't explain why... only guess.