(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 time —i.e. rises to the surface; such and such a thing crops out of what you were saying —i.e. is apropos thereof.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894