I heard a story about one of Sai Baba's devotees fighting off robbers. What happened, and how was Sai Baba involved?

The devotee you are referring to is Kashiram. According to the narrative in Chapter 0, Kashiram was once attacked by Bhil robbers while returning from a market. The robbers tried to steal a small bundle he carried, which contained powdered sugar for feeding ants, a practice dear to him. He fought back fiercely, killing two thieves before being struck by an axe and left for dead. However, he survived. The text explains that while this attack was happening, Maharaj in Shirdi became very agitated, shouting and swearing. The people there understood this to be his way of protecting a devotee in grave danger. Kashiram, having full faith in Maharaj, refused hospital treatment and was brought to Shirdi, where he recovered under Maharaj's grace.