How is it possible for a standalone oblique shockwave to form on the upper surface of a wing? Oblique shock waves form because :

"An oblique shock wave is a shock wave that, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses." - Wikipedia.

(The reason for putting standalone into the title is because the front shock on a lambda shock could be considered standalone)

So for the flow to turn into itself, there has to be an angle, which is usually provided by flow separation. So there has to be something that causes the separation, which is a shockwave. My question is this : Why is the front shock in a lambda shock considered an oblique shock? What causes the separation for the flow to turn into itself? this answer's comments has some good information, but I was also curious why the front part of the lambda shock is an oblique shock.

My guess is that the shock starts out as a normal shock at lower speeds, but as you speed up, it causes separation forming an oblique shock. What I can't figure out is what would make the normal shock (the one 90° to the surface) go away. If it did go away, what would cause the separation for the oblique shock to form? It almost seems like there has to be a normal shock for an oblique shock to exist (in this situation).

(I'm usually bad about doing prior research to questions, but this time I did. If I missed any sources that explained this, let me know!)