They are believed to be sorei (a holy spirit or shoro) that have returned during the period of the New Year (shorei and sorei are the origin of Toshigami, the god of the incoming year) or Obon (the festival of Buddhist All Souls' Day).
正月（精霊・祖霊は年神の起源でもある）やお盆の時期に帰ってきた祖霊（精霊・しょうろう）といわれる。