From my sources deep within the trucking industry comes the following: Apparently, it has to do with axle weight limits for trucks. The amount of weight that a truck can have per axle is decided by the department of transportation in each U.S. state.
When you load a dump truck with gravel, the weight can easily exceed the per-axle limit, so the extra wheels/axles are lowered to make the truck legal. They are raised at other times to improve gas mileage and eliminate wear on the tires.
See How Hydraulic Machines Work for a complete discussion of basic hydraulic principles as well as an in-depth look into log splitters and heavy machinery like shovels and loaders. You will be amazed at what hydraulic machines can do!
Here are some interesting links: