In short, being chronically hypercapnic REDUCES renal blood flow, which results in increased water and sodium retention. This results in a dilution effect for sodium and registers as hyponatremia (SIADH).
Treat with IV saline and monitor to make sure it isn't due to another cause, but usually in a long-standing COPD patient with an acute exacerbation this is what you'll see. Lab values are usually like 125-130 so it isn't really striking but its something to keep an eye on.