adventitious
From Longman Dictionary of Contemporary Englishadventitiousad‧ven‧ti‧tious /ˌædvənˈtɪʃəs◂, ˌædven-/ adjective formal CHANCE/BY CHANCEhappening by chance SYN unexpected —adventitiously adverbExamples from the Corpusadventitious• But the more rapidly an organism learns, the more vulnerable it is to adventitious contingencies.• There was an adventitious contributing cause.• More importantly, the definition of the adventitious population stresses an element of choice in rural residence.