Parkinson's law states that work expands to fill the time available for its completion. As people don't want to look like there're lazy, so they find extra tasks to tackle, even if they are trivial.
A generalised, and more mathematical, version of the law is:
The demand upon a resource tends to expand to match the supply of the resource (if the price is zero)
This generalisation has come to resemble what some economists regard as the law of demand – namely, the lower the price of a service or commodity, the greater the quantity demanded. (Wiki)