I have posed the question on Yahoo Answers :-
So far have this one -
For the same reason you don't put on your parking brake when you put your car in long-term storage.
Molecules are in constant random motion. When two substances are in contact, then molecules from one substance can migrate into the other, and vice versa, creating a blend of the two substances at the interface.
With rubber, we take it a step further. Rubber is just barely a solid. You might say it is a very, very viscous liquid. As a quasi-liquid, it will "flow" right into the rubber in contact with it. All this "flowing" results in a big "solution" of rubber.
It's not so much a chemical reaction, as it is physical process