Failure to elect at the proper election or to appoint within the time fixed by law or the failure of the officer elected or appointed to qualify within the time prescribed by law; the incumbent ceasing for any reason to be a resident of the district or removing residence from the subdistrict; the resignation or death of incumbent or of the officer-elect; the removal of the incumbent from, or forfeiture of, the office, or the decision of a competent tribunal declaring the office vacant; the conviction of incumbent of a felony, as defined in section 701.7, or of any public offense involving the violation of the incumbent’s oath of office, shall constitute a vacancy.