Most people seem to ascribe it to the login change (mid 2012 I think?) but at the same time, Quora was -exploding- in India. This came with it a big shift in the types and relevance of questions. By the start of 2013, India had more users on the site than any other country. As a heavy site user at the time, it felt like it happened overnight. For me this seems more impactful than the login change.