The alignment goes out after a hit due to bending of components. There are three main things that are looked at during an alignment, and a couple of secondary measurements. I will only discuss the main three, which are camber, caster, and toe. The toe and camber are usually the ones that are adjusted when you get your alignment, as the caster is normally set by the vehicle manufacturer. Caster allows for assistance in turning, as it returns the wheel to a centered position. Camber allows for the tires to be placed flat on the ground during a turn, assisting the vehicle while turning. Toe assist in your steering, as it determines which direction the vehicle travels in relation to a straight center line.
When your alignment is off, it is usually caused by your camber, and toe. These two measurements are normally controlled by your suspension (struts, or mounts) and the tie rods (thin metal screw rods). As we all know roads are not the greatest in most areas of the world, so as the vehicle is traveling, under constant vibrations, it will make some of this parts come loose. This allows them to move around, and slowly come out of alignment. Also, air in your tires is a big factor in your alignment, so if your vehicle is aligned with flat tires, it will be a horrible drive when you air them up.
Lastly, as Brian mentioned, wear on the joints will give the suspension movement that contributes to the misalignment of your vehicle. This can either be caused by slow bending of components, or the extra play accelerates the loosening process of the nuts holding all those components in place.