Spirits become earthbound because they forget how to return to heaven. They will linger on earth, keep doing their activities, only to see that no one responds to them anymore because they aren't visible to the living. This will continue for a few while, then the bereaved will light the path to help them reach 'home' on the 49th day after their deaths.
Earthbound spirits are harmless during this period. But over time, they will start losing their own memories... until they finally start attacking the living.