When you are getting close to one of the main scenes of the story, as if you are acting that scene, you might suddenly shift to the present tense. That also happens when you start to describe a place or a person or a situation in the middle of your story.
It depends how long a written story is to me; if it's short, chpping back and forwards between the present and the past can be a bit confusing, but it can bring the reader closer to the story (in theory).