No wrecker, the Aussie loose head has dropped, which means the Scots tight head is suddenly held on one shoulder only (his left) and he pivots round it.
Or he’s driving in.
You can’t tell from a still.
Edit : actually on the shorts thing you point out, more shorts showing on the Aussie loose head also suggests that his hips are higher than his shoulders – which tallies with his shoulders dropping, which tallies with the Scots tight head suddenly having no support for his right shoulder because the Aussie loose head has gone to ground.
But you still cannot tell from a single shot from above.
And look at Laidlaw. A scrum half with the ball at his number 8’s feet does not point at a possible reason for the ref to give the opposition a penalty. Good sportsmanship should be applauded but that would be a step too far.