I'm no scientist so I can't specifically answer your question as to WHY, but I can give you a practical solution to the underlying problem. Just add water. I don't remember how I figured this one out, but whenever I want to render all the fat out of bacon for later use in mayonnaise, hot bacon dressing etc... what I do is cut the strips of bacon (store bought) into 1" pieces. Then I put it in a frypan with 2-3 tablespoons of water over medium high heat. It starts off as a gloppy mess, but as the fat renders and the water evaporates, it eventually produces perfectly browned and crispy bacon bits and beautiful golden bacon grease. Before I figured this out, I would never be able to get enough fat rendered out of the bacon. I'd either burn it or end up with floppy, fatty undercooked bacon and not nearly enough grease. With the water added, I seem to get almost COMPLETE separation of the meat and fat parts of the bacon. I have also tried adding oil to the pan instead of water, but never got the same results as with water.
So I'd say it definitely has something to do with the added moisture in the brined bacon.