I thought it was root rot because the outer layer was sliding off when i touched it - very moist. Most of the small roots broke off or maybe were already broken when I pulled the tree out.

There is a very good chance that the foliage was getting more sun than the pot. I don't have a full window and the pots aren't far enough back to where the sun would hit the entire pot evenly.

The place where I put the pots also had a heat outlet almost directly above so maybe underwatering was also an issue. They wouldn't dry out so I didn't want to water every week.

I just don't understand why this pot was affected and all others, except my gold nugget mandarin, are fine. At least this gives me an understanding as to why it may have died and I can learn from my mistake.

If your feeder roots detached while trying to slip the tree out of container and you say that the outer sheath on the remaining roots rubs off quite easily, then I wouldn't rule out the possibility of root rot. On the other hand it's really hard to believe that it happened to a tree in such a potting mix (in terms of aeration) and your watering schedule...