Never plugged in... ever??

Capacitor's hold a charge.. if it was plugged in, even a number of days or weeks ago.. a small amount could've been stored in the capacitor and released at any random point especially if there was a bulb to take the stored charge.

Even if you didn't plug the ballast in when you bought it.. they test it when manufactured so it could still have a stored charge.
....

.... This is all theory of course.. but i'm sure it could happen...maybe....