Why Singapore?
According to a statement by the Hong Kong government, this is due to similar successes in containing the pandemic, and relatively low case numbers in both cities.
However, the travel bubble will be suspended (for two weeks) after two days if the daily average number of untraceable cases in one week reaches more than five in either city.
If the figure drops on the last day of the suspension period, or any subsequent day, the bubble can resume once more. There must have also been three consecutive days with less than three untraceable cases in both Hong Kong and Singapore, and the daily average number of untraceable cases in one week must not be more than five in both places.