Finishing the Dungeon

Your players skip one door on the first level of a dungeon, for whatever reason. They proceed through the dungeon, level by level, defeating monsters, surviving traps and finding loot. They defeat bosses along the way, and finding the entrances to the new levels proves not particularly difficult. So why do they stop their progress on level four to go back to that level one door? What is it about dungeons that call for a complete cleanout?