This led to many believing that it was a hack, but a statement from Valve then revealed that it had something to do with caching, and that it was not a hack and that nothing was compromised. However if you wanted a more detailed and public explanation, in case you missed the earlier report, Valve has obliged by posting a lengthy blog entry on its website.
According to Valve, there was a DoS attack on Steam which prevented the store pages from being loaded for users. “In response to this specific attack, caching rules managed by a Steam web caching partner were deployed in order to both minimize the impact on Steam Store servers and continue to route legitimate user traffic.”
However it seems that there was a second attack and this is where the caching issues occurred. “This configuration error resulted in some users seeing Steam Store responses which were generated for other users. Incorrect Store responses varied from users seeing the front page of the Store displayed in the wrong language, to seeing the account page of another user.”
Valve then decided to shut the Steam Store down while a new caching configuration was deployed, which is why users weren’t able to log into the store during that time. In any case it has all been sorted and Valve notes that those users whose profile information was shown to others will be contacted.