The recirculation system draws air from the cabin and flight deck under the floor and it passes around the lining of the cargo compartments, where it partially aids in heating the compartments (the forward cargo compartment gets an extra boost of warm air from the exhaust air from the avionics cooling system). This "used" air then flows back to the mix manifold where it gets redistributed and reconditioned.
On very hot days, if the cargo compartment doors are open on the ground, as they often are, the cargo compartment gets hot, and so does the air circulating around the compartment lining. Because this means that hotter than "normal" air is being recirculated back to the mix manifold, the packs have to work much harder to recondition the air. Aircraft cooling can be more effective in these circumstances with the fans off, but in doing so it obviously reduces the volume of air flow in the cabin and flight deck, which slightly reduces the ability of the packs to cool the air (less airflow = less cooling). The baggage handlers probably won't thank you though
