A really good question. I have a friend called Katja who I randomly started calling 'Kaj' at some point many years ago, and I can't remember exactly when. I do sort of know why, and it could be similar to the Beej mystery. There are two syllables in each name (BJ and Katja). Each starts with a consonant, and the first letter of the second syllable is a 'j' sound. So just stick with those dominant sounds and scrap the rest = instant convenient nickname. I also eliminated a t, which makes me lazier than Hawkeye o_O

I hope some of that made sense to you, because it doesn't to me ^_^