Theory:

The lesson "That Sunday Morning" revolves around a one day incident that happened in the narrator's life. She moves from Delhi to Patna, as her father has been posted in Patna. She decides to explore the new place with her brother in the morning. Since Patna had a lot of free space, she explores the city in a cycle. Her brother challenges her to a race and fixes a winning mark, which she accepts.
 
As the narrator picks up speed and levels up to her brother, she comes across a cow in the middle of the road. She applies brakes, loses control and lands on top of the cow. The cow, getting confused by the sudden movement, starts running. The cow makes a dash towards two rows of cavalry officers who were mounted on horses. The horses, who also get startled, run around, thereby throwing the riders down.
 
One of the riders fall in to a ditch, into which the cow also falls after dropping the author near it. The rider gets his pants torn in the process of escaping. The narrator's brother laughs at her initially but is worried about her, as they plan to leave quickly before they are questioned for their actions.