When the pedal point is located at a vertice of triangle ABC, the pedal triangle becomes degenerate. This is seen in the picture below. Notice that the pedal point is located at the same point as vertice B of triangle ABC and vertices S and T of the pedal triangle. Since two of the three vertices of the pedal triangle are located at the same point there is only the point where S and T are and point R to form a triangle and since this is actually only two points they define a line which is called the Simson Line(in green).