It might have started for self-defence, but became insurrectionist once the vikings had been defeated. "Why should we pay fealty to a liege that fails at his most basic duty?"

The terms used are "conjurés/conjuration" (from Google: Middle English (also in the sense ‘oblige by oath’): from Old French conjurer ‘to plot or exorcise’, from Latin conjurare ‘band together by an oath, conspire’ (in medieval Latin ‘invoke’), from con- ‘together’ + jurare ‘swear’.). It seems to imply something more than just gather together for self-defence.