Deep trenches form at the leading edges of subduction zones. If you watch as the pointer moves around the screen on the video and look at the elevaton numbers at the bottom ( they are negative so indicate the depth below sea level) the sea depth reaches 22,000 feet just east of tonga.
Tonga lies on a tiny microplate trapped as a rise between a spreading zone to the west and the subduction zone to the east. Recent volcanic activity appeared 10 kilometers from the southwest coast of Tongatapu, the main Tongan island. There are 36 other underwater volcanos in the vicinity.