What is the reasoning behind a pinched nerve?

Pinched nerve: There is no reason needed. It is a common occurrence that may or may not be symptomatic. Causes include herniated disc, bone spur, stress fracture, slipped vertebra, spinal fracture, tumor, trauma... As you can see, without further information, difficult to make an assessment. In a young person, likely a disc herniation with 80% chance of nonsurgical improvement with pt, nsaid's, and time.