Please note that while I try to answer your question to the fullest extent possible, my knowledge mainly comes from school and general knowledge as well as being a citizen of Austria and thus my views on Austrian identity in relation to Germany may not reflect the general population, although I try to take that into account.
Firstly, the 99% approval rate that you quote (if it even was that high) was in all likeliness an exaggerated number published by the National Socialists Party (Hitlers). While yes, approval was high (something that nowadays Austrians like not to acknowledge) it was not that high. Austria was not allowed to merge with Germany mainly to keep it from regaining its powerful state it was in before and during the first world war.
Like Germany, Austria lost it's monarchy and went through radical change. While Germany was a force, Austria was reduced to a shadow of its former self and so a general sense of helplessness may have played a part as well.
You are right in that nowadays Austrians SAY
We're not Germans. Never ever call us Germans, okay ?!
This seems to more align with the sentiment of "Well, we ARE a sovereign country and have our own culture (that is deeply connected with parts of German culture) while also sharing the language." The forceful disambiguation when separating Germany and Austria seems to stem more from a need for factual accuracy/irritation of being mistaken for "just another part of germany" and does not indicate any strong feelings opposed to Germany or Germans. I will venture as far as saying that we will deny the connection with the same kind of "light irony" when trying to convince ignorant people that NO, we're Austria. In central Europe. Not Australia. Which is on the other side of the globe. This is at least true for younger generations that don't have strong feelings about the Anschluss as other generations may have.
So, since it seems I have not directly answered your question about Austria's general "180° shift" it is because internally, such strong shifts may not have occured. After the second world war, Austria (like every other country occupied by Germany) was, and wanted to be seen as just another victim of Hitlers "Lebensraumerweiterung" (expansion of land to live of for the german people). From this perspective, it seems as though the strong words of distancing from Germany are caused by reflexive self preservation.