What would cause an electrical short that would ruin the battery? on 2003 GMC Safari

The van was electrically dead after driving it a half hour before. When we tried to jump it, it almost caused my buddy's truck to die and over heated the jumper cables. Took out the old battery and when installing the new one it arced and caught the side post on fire. What would cause this kind of short?

Asked by for the 2003 GMC Safari
Sounds like a dead short in the alt, I would get the van to a repair shop to get it looked at. I once had a car with an electrical problem and the guys at Blossom found it.