The English idiom “crop up” is used to indicate that something has appeared suddenly and unexpectedly. You may also hear people say that something has “come up,” which has a similar meaning. The idiom has been used in this sense since at least the 1800s, and evidence seems to suggest that it may be even older, with the earliest dated references to cropping up appearing in the 1600s.
Although the term "crop" is associated with plants by many modern English speakers, the origins of this word reference the original meaning of "crop," a swelling or protuberance. Originally, this term was an entry in the geological lexicon. Geologists would say that something was “cropping up” in the literal sense, as in an outcrop of rock. Rocks do periodically crop up very suddenly as the landscape erodes or is rearranged by an earthquake, and outcrops are especially common in mines, where miners find harder rocks backing softer materials.
There are some intriguing alternate suggestions for the origins of “crop up.” Some people have suggested, for example, that the term is a reference to crops which volunteer where they are not planted, as in “some potatoes cropped up in the carrot patch.” Others have claimed that the term refers to the small stones, roots, and other debris which would come up during spring plowing, when farmers prepared the land for planting. However, etymologists strongly believe that the term started in geology.
As some publications from the late 17th and early 18th century suggest, the idiom was quickly adopted in a metaphorical sense, with people talking about situations and events which had “cropped up” exactly like outcrops of rock. The first written instance of “crop up” in this sense occurred in the 1800s, cementing the idiom in the English language.
There are a number of ways to use this idiom. For example, one might say “something has cropped up at work, and I will need to stay late,” using the idiom to suggest that an unexpected event has forced a chance of plans. This idiom is sometimes used when someone is making excuses for not doing something, and it has acquired a somewhat suspect meaning for some people, with “crop up” being taken as a euphemism for “I didn't feel like following through on the plan.” One might also say “more students crop up in the literature program every year,” referencing the idea that this term refers to new appearances.