Nearly half a century after the discovery of aurora substorm, the origin of this fundamental phenomenon in space has remained a subject of active research. It has been widely believed that the onsets of aurora substorms originate from plasma processes in the magnetotail. However, the exact mechanism for the trigger of an aurora substorm onset has not been well understood. Two major candidates for the trigger mechanism, magnetic reconnection in the middle magnetotail and ballooning instability of the near-Earth plasma sheet, are examined in light of recent progresses in observations and theory analyses. A new scenario is proposed, in which an axial tail instability can be facilitated by the ballooning instability in the near-Earth magnetotail and lead to the subsequent magnetic reconnection and substorm onset.