24 synonyms for unexpected such as adventitious, abnormal, and abrupt. Unexpected is an adjective, and its antonyms are expected and predictable.
Abnormal, abrupt, adventitious, astonishing, backasswards, chance, extraordinary, fortuitous, not bargained for, out of the blue, serendipitous, startling, sudden, surprising, unanticipated, uncommon, unforeseen, unhoped for, unlooked for, unplanned, unpredicted, without notice, and without warning.