Some fanfictions might have a plot where Harry accidentally becomes a lord. Maybe he stumbles upon a magical artifact or solves a riddle that was tied to an old lordship. This unexpected lordship then throws him into a world of mystery and danger. He has to figure out what it means to be a lord, deal with the jealousies of other wizards, and also use his new position to protect his friends and Hogwarts.
One common plot is that Harry is made a lord due to his heroic deeds. The wizarding community decides to bestow upon him a lordship as a sign of respect and to give him more influence in the fight against the dark forces. This often comes with a new title, a manor or some sort of property, and a seat on important wizarding councils. It changes his status from just a student to a significant figure in the wizarding world overnight.
Well, in some 'Harry Potter Unexpected Lordship' fanfictions, Harry might discover a hidden lineage that makes him an unexpected lord. For example, he could be related to an ancient wizarding family that was thought to be extinct. This could lead to him having new powers, allies, and enemies. Another plot could be that he inherits a lordship through some sort of magical will or prophecy, and he has to navigate the complex politics of the wizarding world as a result.