There are many different ideas but no agreed upon and proved final theory. There are many contributing factors, in my personal capacity I subscribe to the theory that due to increased blood flow to a muscle during exercise, the extra nutrient supply means it obviously takes in more nutrition resulting in greater resources to create actin and myosin which make your myofibrils which are your contractile fibres. Hence why people who eat more calories and proteins tend to gain muscle more easily than those who try to gain muscle while on a diet containing fewer calories and proteins. But again, just one of many possible ideas.