At the end of Aqua and Mickey's battle with Ventus-Vanitas, the X-Blade starts to break apart and sends out shards of light that fade in the outline shapes of KEYHOLES. Could the completion and subsequent destruction of the x-blade, being the physical manifestation of Kingdom Hearts, be the reason that the KH1 worlds' keyholes were open in the first place? If the X-blade did open up every worlds' keyhole before its destruction, it would be what allowed the Heartless to start devouring worlds for the next decade until Sora's first journey.