Most common cause for that is the actual battery cover contacting the battery post and or battery cable in of itself. Thats a sure fire way to zap your battery.

side note.. if your battery post melted.. your battery had a good charge. you shorted, or grounded your battery onto itself causing that sucker to bake right at the post.
check the back of your battery cover, see if you have any burns, or crustyness in there.