Scientists think the brain's prefrontal cortex processes experiences, using sensory input from the eyes, ears, nose and mouth, sorts them into categories, and tags the various memory fragments with specific associations (smells of home, friends from camp, bugs, a pet, for example).
When a memory cue comes in, the brain searches its circuits for related fragments and assembles them like a jigsaw puzzle. Some fragments bring associated fragments along, which is why one old memory often leads to others. Tastes and smells are particularly evocative