[This is the culprit](http://meta.stackexchange.com/users/-1/community). The bot deletes zero-score unanswered questions (with fewer than 2 comments) after 365 days. The spike that you observe is not a spike in questions posted, but a reverted wave of questions _deleted_. In the March snapshot, this wave of deletion is in March 2014 (around week 10). In the current snapshot, it is in July 2014.

Removal of abandoned questions after 365 days [is the 3rd most common deletion reason](https://meta.stackoverflow.com/a/296609) on Stack Overflow, trailing only the deletion of questions and answers by their authors.