The answer is organisms, oxidation & water. Organisms such as plants have their roots penetrate the cracks in the rocks. As the roots grow and become bigger they put mechanical pressure on the rocks and the cracks increase in size and the rocks are split. Additionally, organisms secrete organic acids, as wastes, that chemically react with minerals in the rocks hence causing weathering.
Oxidation is a chemical reaction that changes the property of minerals by makes them prone to weathering. An example is the oxidation of iron in the rocks to iron III oxide which is brittle.
Water participates in chemical reaction by dissolving minerals, such as limestone (CaCO3) from the rock hence leaving the rock porous and more predisposed to physical weathering agents.