What is the scientific reason avacados, bananas, etc, ripen faster when placed in a paper bag?

This is because bananas and avacados emit ethylene gas during ripening. This natural ripening hormone speeds up the process of turning those fruits into sweet-as-candy delights. By putting the fruits into a loosely closed paper bag, the ethylene gas surrounds the fruits, helping with the ripening process.

Ethylene acts physiologically as a hormone in plants. It stimulates the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves.

Avocados, Bananas... they give off ethylene gas as they ripen. The gas helps to ripen them which in turn produces more ethylene. Keeping them in a paper bag traps some of the gas and this helps speed up the ripening process.