The reason is adding the "-i" for the plural (ananas - ananași).
These are some basic mutations that happen in this case:
-s/-ș: pas - pași (step - steps)
-t/-ț: deștept - deștepți (smart --- adjective)
-d/-z: bard - barzi (bard - bards)
-st/-șt: poveste - povești (story - stories)
-str/-ștr: astru - aștrii (star - stars)
-sc/-șt: broască - broaște (frog - frogs)
-șc/-șt: pușcă - puști (rifle - rifles)
Of course, there are some exceptions (e.g. nădejde - nădejdi (hope - hopes)), but these should cover a lot of words.