If I were to speculate on the reasons for the Wallabies' complete lack of form in the back part of the season it would be the usual, sure: depth, no third tier, weird selections and player management. But there's one that almost no one mentions... Geoff Parling. Parlling left the Wallabies to take over Leicester after the Lions Tour. After that, the Wallabies had one more good match in them - Ellis Park. After that the decline set in, especially in the lineout and maul, which were Parling's areas. I believe the Wallabies' slide predates the Bledisloe and they never recovered.
This is also why there were some weird 2nd row selections after that. Whenever Skelton was not available - and sometimes he was overlooked even when he was available - LSL could never get a look in. We really needed a line bender/breaker, especially since Valetini was out of form.