The phrase "crop up again" refers to something unexpected or unwanted occurring repeatedly. Antonyms for this phrase would be words or phrases that indicate an absence of repetition or unexpectedness. Example antonyms for "crop up again" include consistency, predictable, routine, unchanging, persistent, etc. These words denote a sense of stability and continuity. Another set of antonyms would be words that suggest the opposite of the negative connotations of "crop up again," such as rare, seldom, sporadic, infrequent, or occasional. These antonyms convey a sense of something happening once in a while or at intervals with positive connotations.