Why do people with COPD tend to have fluctuating weight gain and fluid retention?

Steroids. Patients with COPD do not normally do that unless they are on and off steroids, pills or injections.
See below. In patients with copd, the right heart is strained because it has to pump blood into the damaged lungs. When the right heart cannot pump the blood properly, that causes fluid back-up/retention which in turn causes weight gain.
Heart strain. COPD can put a lot of strain on your heart , causing heart failure. When you have heart failure, you retain fluid.