"cropped up"' implies that it sort of snuck up on you without you expecing it. Whereas something has happened, you may or may not have known, but either way it happened.

Katrina: "Cropped up" is merely an idiomatic expression which alludes to things "happening" such as the growing of plants, which are referred to as a farmer's "crop" or "crops". You can use this idiomatic expression to indicated that something has happened. However, the literalism, "happened" is a much clearer communication. Idiomatic expressions can be regarded as a bad habit. Literal communications are always clearer to any listener or reader. . .