The Kermadec Trench quakes were at a shallow depth (about 12 miles, or 20 km). And this could indicate an increased chance of a tsunami. Both the US-based National Oceanic and Atmospheric Administration (NOAA) and the National Emergency Management Agency of New Zealand issued tsunami warnings for the islands in the southwest Pacific.
About Earthquake Swarms
In October 2020 a similar earthquake swarm occurred in California. These are rare, but not unprecedented occurrences.
Recently scientists were able to determine that oil and gas wastewater disposal wells or the underground injection of fluids can cause earthquake swarm phenomena. This was studied in parts of Oklahoma in 2009.
They also know that the earthquake swarm moves along faults, like volcanos or oil and gas geothermal operations. The swarms move at differing rates of speed depending on the environment where it occurs.