unprovoked
adjective uk/ˌʌn.prəˈvəʊkt/ us/ˌʌn.prəˈvoʊkt/
（令人不快的言行）无端的，无缘无故的
If an unpleasant action or remark is unprovoked, it has not been caused by anything and is therefore unfair.
an unprovoked attack on her character 对她人格的无端攻击
Synonym: motiveless