Many rollerguns have their bands under tension when fitted on the gun and before you even cock the gun. Rubber under tension is attacked by oxygen in the air, so in order to prolong the life of bands this tension needs to be removed when the gun is not being used. You can see an example of this attack on most speargun bands where you get a crack running around where the wishbones are tied into the bands as this squeezed down rubber is always under tension. Nothing that you can do about it, although keeping the bands in an air-tight container can prolong their life, however from new oxygen is being absorbed into the rubber unless it has a protective coating to slow the process. Antioxidants can be added into rubber compounds, but this is something more commonly done with automotive rubber products such as seals, hoses and tires. Sunlight also degrades rubber, hence bands should be stored out of direct sunlight even when left on the gun and more so with rollerguns when the bands are curved around the muzzle rollers.
On Seal's (Kosta) rollergun he had a pin that could be pulled out which anchored the bands on the gun, hence with this disengaged the bands could be allowed to fully relax or even be removed.