Hi Bill,
Let me have a try at explaining: The "liquid" magma below the Earth's crust is to be compared to porridge in a pot on a hotplate. Just the scale (space - viscosity -> time) is different. Due to the thermal gradients (complicated by chemical reactions and magnetic forces), it rises at certain places, thus in other places, it has to sink. The plates of the crust are like dried flakes of the porridge. They cannot follow the downward movement of the highly viscous liquid, and are thus squeezed together.