Why does post surgical breast reduction swelling go up and down so much?

Surgery interference. Surgery may interfere with normal processes of fluid drainage and may cause restrictions or limitations in the ways the body's normally treats swelling. Inflammation in the initial post operative period causes marked swelling which can be followed by scar tissue formation that produces swelling lasting 6-9 months. Abnormal or one sided swelling should be evaluated by your surgeon.
Post op swelling. This is totally normal. Increased activity, hormonal fluctuation, even bending over more (gravity) can alter swelling. It takes months for this process to resolve, as the normal pathways of lymphatic fluid drainage reestablish themselves through normal healing.
tissue healing. Postoperative swelling can be found in nearly every surgical procedure. In the case of breast reduction the tissue is rearranged and normal lymphatic drainage may be interrupted while microvascular patterns reestablish themselves over the course of months. The healing part of the body is also more permeable and thus during periods of water retention will swell to a greater degree than other parts.