I am not an expert, but I can tell you from my own experience that while on a Water Fast you are still consuming nutrition from you body. What your body is consuming is fat cells and other sources of protein needed for your body to function correctly. Parts of the cells are not completely consumed and thus are wast products that need to be expelled every few days.
Two years ago I undertook a forty day fast and, if I remember correctly I had three or four BMs in that period of time.
It is natural and normal to have bowel movements when fasting, just not very often and not very much.
Like I said I'm not an expert and I can only speak from my experiences.
Best of Wishes