Created obstruction.

The Stormers throw into a line-out. De Kock Steenkamp catches the ball and the Stormers gather around in maul formation. The Brumbies make no contact.

The Stormers work the ball to the back of their formation which would like to be maul but does not manage to be one because the Brumbies will not play ball.

When the ball is moved back from Steenkamp, Stephen Moore, a lone Brumby charges into JC Kritzinger who is nearest to Moore.

The referees penalises the Stormers for obstruction. They had moved the ball to the back of their formation before the Brumbies had made contact. That is pretty rudimentary obstruction.

10.1 OBSTRUCTION
(c) Blocking the tackler. A player must not intentionally move or stand in a position that prevents an opponent from tackling a ball carrier.
Sanction: Penalty kick
(d) Blocking the ball. A player must not intentionally move or stand in a position that prevents an opponent from playing the ball.
Sanction: Penalty kick

There is no obligation on the Brumbies to form a maul. Brains won them that penalty.