It's a shockwave caused by the first bit of supersonic material to hit the air and forms at the leading edges.
Totally agree, however what makes the material supersonic ..... the speed of the local airflow going over it, round it etc. Therefore it will cause a shockwave.... well thats what I was always taught and what I remember from running supersonic wind tunnels. But I may be wrong, it was all a long time ago.....