https://www.gamespot.com/articles/fortnite-season-6s-floating-island-is-moving-aroun/1100-6462155/

The mysterious island floating above Loot Lake in Fortnite has started moving, though to exactly what end remains unclear. For those that haven't been keeping up with the latest happenings in Fortnite, a strange purple cube appeared in the game and plunged itself into Loot Lake just before the start of Season 6. Once the season kicked off, however, it floated into the air, taking a chunk of land up there with it.

Currently, the cube and the attached island seems to be just moving between locations on the Fortnite map. Fans have speculated that the it's moving between the runes that the cube left on the map towards the end of Season 5, before it dropped into Loot Lake.