tl;dr: They're claiming that it was to "prevent countries with tariffs from shipping through there to avoid tariffs."

The United States alleged the islands exported more goods to the United States than they imported, an allegation that appeared to be calculated from incorrect trade data. An analysis of U.S. import data and shipping records by The Guardian indicated some shipments were incorrectly labeled as coming from the remote islands instead of their correct countries of origin. According to export data from the World Bank, the US imported US$1.4m (A$2.23m) of products from Heard Island and McDonald Islands in 2022, nearly all of which was "machinery and electrical" imports.[39]

https://en.wikipedia.org/wiki/Heard_Island_and_McDonald_Islands