Were you to keep fasting (or to eat a very low carbohydrate diet) you would burn off all the stored glycogen that can be released and the peeing would stop. At that point, your body would switch over to burning first ketones and then, after a few weeks, free fatty acids. But since with intermittant fasting we mostly go back to eating carbs the day after a fast, our glycogen stores get recharged and we put back on the weight that is due to the glucose bonding with that extra water as glycogen is formed.
This cycling back and forth through glycogen burning and storing is the one thing I don't like about intermittant fasting. With low carb dieting you get past it for a few days and then that's that. No more wearing a path to the bathroom!