* Weathering: The harsh desert environment with its extreme temperature swings, lack of moisture, and strong winds causes significant weathering.

* Thermal Shock: Rapid temperature changes cause the sandstone to expand and contract, leading to fractures and disintegration.

* Salt Weathering: Evaporation of water leaves behind salts that crystallize within the sandstone, exerting pressure and breaking it apart.

* Wind Erosion: Strong winds can carry sand particles that act like sandpaper, eroding the sandstone's surface and weakening it.

* Lack of Cementation: Sandstones form when sand grains are cemented together by minerals dissolved in water. In arid environments, water is scarce, leading to less cementation and weaker sandstones.

* Type of Sand Grains: The type of sand grains that make up the sandstone also plays a role. Some minerals are more prone to weathering than others. For instance, feldspars, which are common in many sandstones, can be easily broken down by weathering processes.

* Geological Age: Older sandstones have had more time to be exposed to weathering, making them more likely to be crumbly.

It's important to note that not all desert sandstones are crumbly. Some are quite hard and resistant to weathering, depending on the specific geological conditions and the minerals present.

Overall, the combination of these factors contributes to the crumbly nature of many desert sandstones.