When and why use triangle solder joints
Please note a little notch (adjacent to "R7"), a narrow slot in the thick trace. These special-shape traces are used to minimize (separate) effects of one high-current path from another high-current path, or maybe from a sensitive signal.

The triangles are deliberate solder mask openings to allow a solder build-up, to beef-up the trace near the joint, to compensate for reduced current carrier capability of the PCB traces at the junction point.

Since many of the through-hole pins are of a large diameter, there is often little annular copper remaining to form a current path on it's own, so the solder itself is used to provide increased current capacity at those locations.