Carole of Sussex has one of the two most likely explanations. Cracking corn was one of those onerous tasks on any farm where corn was raised or used.

The other is the old meaning of talk or idle discussion, and may be from the word craik, crake, craic- to boast or brag, or engage in idle talk. The word is Old English, but has survived in Scotland and Ireland (there is a thread on this). It may survive as "crack wise," formerly (locally still?) used in the States.

Crake, imitative of the grating sound of some black birds, is used for the call of rails, crows, etc. Old Norse Kraka is a crow or raven.