The film depicts the story of Oscar Grant III, a 22-year-old from Hayward, California, and his experiences on the last day of his life, before he was fatally shot by BART Police in the early morning hours of New Year's Day 2009. The movie begins with the actual footage of Oscar Grant and his friends being detained by the BART Police at the Fruitvale BART station on January 1, 2009 in Oakland California at 2:15 am before the killing.
The film shows scenes of him and his girlfriend Sophina arguing about Grant's recent infidelity. It later shows Grant unsuccessfully attempting to get his job back at the grocery store. He briefly considers selling some marijuana but in the end decides to dump the stash. Grant later attends a birthday party for his mother and tells her afterward that he will take BART to see fireworks and other New Year's festivities in San Francisco.
On the return train, Katie, a customer at the grocery store where Grant used to work, recognizes Grant and calls out his name. A former inmate, from when Grant was in prison (shown in an earlier flashback), then recognizes Grant and attempts to assault him, starting a disturbance that leads the BART Police to intervene. Amid the chaos, Grant's girlfriend calls and asks where he is; he assures her he is fine. In the end, after Grant is restrained on the station platform, a BART Police officer shoots him in the back. Grant is rushed to a hospital but later dies.
In the post-credits scene, title cards show that Grant's death sparked a series of protests and riots across the city and that the incident was recorded by several witnesses, either by cell phone or video camera. The BART Police officers who were involved were fired and the one who shot Grant was later tried and found guilty of involuntary manslaughter, claiming he mistook his gun for his Taser, and served an 11-month sentence. There is also footage of a gathering of people celebrating Grant's life on New Year's Day 2013 with the real-life, much older Tatiana (Grant's daughter) standing among them.

Question: Why were the police called to the BART station the night Oscar was killed?
Answer: Oscar and a former fellow inmate were fighting
Alternative answer: a former inmate assaulted Oscar