I'd always assumed that harrowing came from the farming implement, the harrow, but it seems that it probably comes from the verb to harrow 'to plunder, sack', which is related to to harry 'to disturb, distress' (no Royal Family jokes, please). By the way, the egger in California's governor's name, Schwartzenegger, refers to the same equipment (modern German eggen 'to harrow': the black harrower / ploughman; the n belongs to the previous word of the compound and not the latter as it's oftentimes assumed. Of course, I suppose one could argue that the adjective really came from the London borough and its associated public school.

So it seems that, etymologically speaking, we have two harrows that have collapsed into one. Just like the two cleaves, one meaning "to stick" and the other meaning "to split", from different roots.