late 15c., kremelen, "to break into small fragments" (transitive), from Old English *crymelan, presumed frequentative of gecrymman "to break into crumbs," from cruma (see crumb). Intransitive sense of "fall into small pieces" is from 1570s.
The -b- is from 16c., probably on analogy of crumb (where it also is an unetymological intrusion) or of French-derived words like humble, where it belongs. Related: Crumbled; crumbling. Old English gecrymman yielded Middle English crimen "to crumble" (transitive).
As a noun, from 1570s as "a fragment," from 1947 in cookery as dessert dish with a crumb topping, British English. "The technique itself seems to have been a product of Second World War culinary making-do" [Ayto, "Diner's Dictionary"].