The reason that it occurs in the context of cardiac failure is simple. In this condition, by definition, the heart isn't able to adequately perfuse the tissues, and those tissues include the kidneys. At night, however, such patients are obviously (like the rest of us) lying down. With the entire body flat, fluid that has pooled in the extremities during the day finds it easier to return to the heart, and consequently the cardiac output at night in these patients is slightly increased. All this means that the kidneys can filter more blood, but at the expense of making more urine - all at night.