I would wonder if this were a bad cable and/or a toasted alternator that would cause this. Either way, something is connecting that positive cable to ground.

Check the positive battery cable and surrounding area to see if the wire grazed something and maybe melted the insulation off and grounded. If this is the case, you should see evidence of this where it made contact and arced.

If the positive cable looks like it didn't graze a ground (just all the insulation burned off uniformly), I would be testing the alternator and making sure that positive post doesn't have a path to ground through the alternator itself. I would take it in to be tested or at least use a multimeter to see the resistance from the alternator post to ground...should be infinite (open) I would think.

Good Luck!