Mixing of non-past/past in narratives


In the ASK Graded Readers, which present beginner-appropriate short stories,
there is a frequently occurring pattern of writing with ~ます during the 'setup'
of the story, and then switching to 〜ました when something important happens. The
sentence containing the tense switch often starts with ある日 or similar. The
story then resumes using the 〜ます form once we leave this major event.

What is happening with the tenses here--why is the story told partially in
non-past and partially in past? Is it typical for Japanese stories to be told
in this way, and is this pattern also used in spoken Japanese?

