Wave clouds arise by a disturbed atmosphere in the form of mountain waves. Mountain waves propagate within the atmosphere as a stable oncoming flow oscillates vertically as it is first forced over a mountain range and to its leeward side. As the air rises and falls downstream of the mountains, it cools and rewarms over and over again. If enough moisture is present, clouds form at the tops of these undulations due to cooled air masses at each crest.
(It’s this rising and falling airflow that create these clouds that glider pilots also utilize for flight!) In this case, The clouds form a triangular wake behind Tristan as the mountain is considered a single disturbing force to the oncoming stable airflow. Think of the shape of a single boat’s wake on a smooth lake.
As an island, Tristan da Cunha itself is quite interesting. It is the most isolated inhabited island on Earth, over 2000km from the nearest continental mass. The island has no airport (imagine living there, or even visiting!). As it is located far from any fault line, it is thought that a hotspot led to the creation of this island and others in its archipelago.
Check out this paper for the dynamics of this effect.
For another interesting cloud formation caused by islands, check out this paper.
For an idea of what life is like on Tristan, click here.
2016 Sentinel-2 (ESA) image courtesy of the U.S. Geological Survey.