Those of you who have thrown out their backs can attest, a pulled muscle can be more painful that a bone fracture or even than giving birth.
When you throw your back out, you are spraining or straining the muscles in the hips. A sprain or strain is a microscopic tearing of the muscle fibers. It is akin to fraying a rope. The muscle is made up of thousands of muscle fibers. When you pull or sprain or strain a muscle, you are tearing some of the fibers of that muscle.
And when that happens, the rest of the muscle reflexively contracts, to prevent further tearing. This is the start of a trigger point.