E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Crop Up (or) Out.
To rise out of, to appear at the surface. A mining term. Strata which rise to the surface are said to crop out. We also say, such and such a subject crops up from time to timei.e. rises to the surface; such and such a thing crops out of what you were sayingi.e. is apropos thereof.