The story opens with scene between Character A and Character B having a row about a minor car accident. It then moves on to Character A going shopping for food for a dinner party Character A is throwing to impress the boss. The dinner party goes wrong, leaving Character A feeling sad/angry/relieved/whatever - The End.
So, is the focus on Character A and Character B, or on the dinner party and the consequences? The reader feels cheated because they felt directed to focus on Character A and Character B as they were present right at the start, but Character B never turned up again and the accident had no purpose for the rest of the story.
What should have happened is that Character B has a crucial role later in the story - perhaps they could have turned up at the dinner party as the boss's partner and that's why it went wrong.
Nadine Gordimer wrote, "a short story is a concept that the writer can 'hold', fully realised, in his imagination, at one time." Everything in the story should tie in with the main focus - the under-lying concept - because, unlike in a novel, there simply isn't the space for digressions.