They hadn't forgotten their ability in the previous matches. They were losing the momentum of the matches by losing the wickets frequently in the first two ODI. Australian batsmen had failed to show patience in 1st and 2nd matches. The target wasn't too big for Australia. They will play the next ODI series against India. It will be quite tough for them to win against India if the batsmen fail to show the patience.
Although my question was a rhetorical question, you are also right because at the same time, I think that not every team has the same momentum all the time, and maybe that is what happened with the Australian team, especially with their batting line-up, in the first two matches of the South African ODI series?
Actually, my bigger question than how the Australian team will perform in the Indian ODI series is how they will perform in the T20 series against New Zealand. Will New Zealand be able to maintain the momentum they had in their last match in the T20 series? And if they can't, then just like you, I will say here that there is a possibility that Australia will lose this T20 series as well.