As a Lulanti, Buzz Buzz was one of Queen Flora's top agents. As such, when Ness was affected by the Gluttony Beam, Queen Flora entrusted Buzz Buzz with curing Ness and bringing the timeline back to normal using the Reversal Beam. To do this, Buzz Buzz must bring Ness to the top of Lookout Hill in Onett, which is where the meteorite will crash in the future, harnessing the power of the full moon, cast the Reversal Beam on him, and the moon's power allows the timeline to heal, as well as Ness's Gluttony Beam condition. However, due to Eternal Forest being so tough to navigate, that would mean that Buzz Buzz would have to escort Ness there for a long time. With this in mind, Queen Flora transforms Buzz Buzz into a dog so he is more comforting to Ness.
Obviously, this was highly threatening to Giegue's plans. So when Giegue heard of this, he sends a Starman to intercept Buzz Buzz's time travel.