He is renting a small room in a very busy/crowded caravan encampment. He hears a scream. He runs out and engages six men in a fight. Nobody else runs out and piles in, even though there must be hundreds of people within screaming distance... I think you have answered your own question. Your character is unusual compared to everyone else in the camp. It isn't a given that a guy in your fictional world will run out and be a hero. Hence readers wanting more insight into why he helps.