Could sound confusing but its really simple: When a triangle is forming under perspective, some times one of it's waves instead to be a (the most commonly accepted structure inside it) forms a new smaller triangle.
This is more common with waves c, d or e.
In this case we are witnessing a triangle as wave D) with all it's corrective structures ( A,B,C,D and E) inside it. Once finished the triangle that works as extension, the rest of the pattern continue as usual.
As with extensions in other structures in a triangle, the pattern usually do not show more than one extension (but this is a guide and not a rule).
What we should expect from here is more decline to complete the 5)) wave that is now forming.