Short films usually take place in everyday situations and places. This is done so that it is easier for the audience to relate and no time is wasted trying to explain the setting or events that are happening. eg. Someone waking up and getting ready is an everyday activity which everyone is used to, briefly showing this will immediately idicate to the audience what is going on without showing the lengthy process, however, if the short film was about an alien apocolypse this would have to be explained somehow before or during the action takes place which could be too confusing for a 5-10 minute film.
Because the events that take place are usually ordinary things, short films often include a twist which keeps the audience captivated. For example, 'The Black Hole' is a short film showing a normal day in an office until there is a twist that the man walk through walls etc by walking through a black cirlce on some paper.