So the dictionary of intermediate japanese grammar has a whole section on this topic: Tense switching.
I took some important excerpts from there:
DOIJG
"Usually a series of past events are narrated in the past tenses. But primarily in written Japanese discourse that narrates a past event, past tenses often switch to nonpast tenses. [β¦] Logically speaking, the author could have written every sentence in the past tense. Nevertheless, the author sometimes used the past tense and sometimes the nonpast tense. [β¦] [The nonpast tense] sentences describe a circumstance that surrounds Shigo, the main character of the story. The rest of the sentences [in the past tense] describe [β¦] whatever Shigo did or felt in the given circumstances. To put it in general terms, tense switching is a strategy available to the writer to differentiate a stage and a set of chronological events that occur in that stage. [β¦] . So, important, dramatic information is described in the past tense, whereas relatively unimportant circumstantial information is described in the nonpast tense. The use of such nonpast tenses has an effect of creating a vivid sense of immediateness for the reader.
Principle of Tense switching:
A part of a past event (often a state rather than an action) can be described using the nonpast tense, if the writer perceives it to be relatively unimportant circumstantial information that has no dircet bearing upon the major story line."
I edited it down a bit but that is the basic gist explained there. Hope this helps your understanding.