Garo
Boy Wonder
Time loops are created when a substantial amount of energy is amassed and released, creating an "entry event". An entry event is the transmission of mass or energy into the timeline that was not previously present. In this case, energy was introduced into the timeline, creating the vibrations that made the Song of Storms. The entry event coincided with Young Link's entry into the Windmill as part of his traditional exploration of the new town he was introduced to. Guru-Guru, not able to trace the source of wayward vibrations, attributed the song to Young Link. When Adult Link enters the Windmill, Guru-Guru explains that he heard the song "from that boy", even though the song was actually vibrations from an entry event. Thus Adult Link learned the song. Then by use of the passage created by the Pedestal of Time, he was able to go back in time and play it for Guru-Guru, sealing the time loop created by the entry event.
What caused the entry event, however, is the real source of debate. My bet is that it was Majora. Majora needed two things - 1, the Song of Storms, and 2, Link - in order to play his game with Link since the other Masked Children ignore him. Since Link was preoccupied with saving Hyrule, Majora needed to guide Link along the way so he could find his way into Termina. So Majora - a being possessing incredible power - created the entry event.
Did I just make all of that up? Yes. But it was fun.
What caused the entry event, however, is the real source of debate. My bet is that it was Majora. Majora needed two things - 1, the Song of Storms, and 2, Link - in order to play his game with Link since the other Masked Children ignore him. Since Link was preoccupied with saving Hyrule, Majora needed to guide Link along the way so he could find his way into Termina. So Majora - a being possessing incredible power - created the entry event.
Did I just make all of that up? Yes. But it was fun.