Él estaba justo detrás de mí

Él estaba justo detrás de mí

As preterit and imperfect tense is concerned, this question "Él estaba justo detrás de mí.". Why it's using estaba instead of estuvo? I looks up all the rules in the book regarding preterit and imperfect tenses and I just can't seem to differentiate them. Thank you!

It is most likely that this sentence is a part of a story being told. The action that happened in the past (preterit) most likely occurred in another sentence, or in an upcoming sentence. This sentence is 'setting the stage' for that other action that happened in the past.
The imperfect is used for narratives about the past, explaining things that were in process when something else happened (or was happening, or was about to happen, in the past).

There are other reasons to use the imperfect, but this sentence is describing an action that was in progress, in the past.