See clip from 03:08 on counter
I believe that Etzebeth does two things wrong.
Firstly, he does not bind onto the maul when he joins it.
Secondly, he collapses the maul when he eventually does bind onto the red No1
Law 16
7. Players joining a maul must:
a. Do so from an onside position.
b. Bind on to the hindmost player in the maul
Sanction
enalty
11. Players must not:
Intentionally collapse a maul or jump on top of it
Sanction
enalty
Why did the ref award a scrum to SA? Because he did not regard the actions of Etzebeth as illegal, so the maul ended unsuccessfully (collapse and ball not available to play) resulting in scum to team not in possession at the time that the maul began i.e. SA.
17A maul ends unsuccessfully when:
a. The ball becomes unplayable.
b. The maul collapses (not as a result of foul play).
One question from me: What if a player joins the maul legally (from onside position and bound) then loses binding once tied into the maul. Can he/she then reach over the top and interfere with opposition ball carrier, as per Etzebeth's actions in the middle of this maul?