I had to go back to our minister's teaching for this one. I've listened to his entire sermon on Melchizedek and he states that he was a judge because "he was involved in defeating the enemies of that day". I guess this is applying the concept of a Judge as would be found in people like Gideon, Deborah, Samson, etc. If you are interested in a better explanation, I'll ask him for something more concrete than this.
If you are interested in hearing the teaching go to:
http://www.rosh-pinah.org/teaching.html
Play the November 12, 2005 teaching, which starts around the 8:00 minute of the track.