why s'est occupée

Question

S'est occupée is passé composé.  This is a single action completed in the past.  But the text describes an action over the years.  Why not s'occupait?

Accepted Answer

Carl, 
It is not necessary for there to be only 1 event for passé composé to be used, it can also used to describe what the speaker perceives to be a series of individual defined events, rather than an ongoing vaguely continuous ‘thing’ or ‘habit’.
Additionally, and importantly, imparfait is not used for things true in past and still true in present - that is the ‘domain’ of passé composé. It is clear from this story that Grandma is still a cat lover and that the ‘looking after of cats’ continues, hence passé composé is indicated.
See attached excellent video from Hugo Cotton - specifically about the 14 minute mark, advanced uses, but earlier discussion about 7 minute mark starts to introduce this distinction. 
The clip is in comprehensible (relatively slow) French, but subtitles available. Worth watching the whole video. 
https://youtu.be/3rpQ5xeFneg

Answer

Thank you for this detailed explanation