Reuters has a report about it
https://www.reuters.com/world/europe/austrias-kurz-says-stepping-down-chancellor-2021-10-09/

Kurz led one of the less anti-Russian governments in the EU and the scandal which brought him down - though as is often the case almost certainly factually based - looks to me to have been planted in order to bring him down. Note that it was his coalition partner the Greens, who are like Greens everywhere in Europe strongly pro-EU, who who led the attack on him.

To put it simply, I think what we are seeing is a systematic clear-out of 'unreliable' EU leaders, 'unreliable' in the eyes of the Euro-Atlanticist leadership that is. We are seeing an increasing attack on Orban in Hungary and on the Polish government, and Kurz - who is possibly perceived as a potential ally of Orban's and the Poles', at least on some issues - therefore had to be shunted out. So why not publish.a scandal to get rid of it.

As to why this purge of 'unreliable' leaders is now underway, I suspect that it is connected with Angela Merkel's imminent departure from the scene. Whilst she was around she could be relied upon to keep them under control. With her gone clearing the decks of unreliable elements becomes more of a priority.

Having said that, Kurz for the moment looks down rather than out, and it may be that he will be able to claw his way back, though I wouldn't count on it