The number of clicks or impressions could have changed sharply for the following reasons:
An abrupt spike or dip in activity could be caused by any changes you recently made to the campaign settings.
Even minor changes in ad texts and campaign settings (time, impression region, strategy), as well as changes in bidding can influence your contextual advertising statistics. Of particular importance are changes to keywords, as they are the defining condition controlling ad displays. For example, adding a popular phrase to the list of keywords can cause a sharp surge in the number of impressions and clicks.
Content ads are displayed in the ad networks (Yandex Advertising Network and ad exchanges) if the advertising content corresponds to the content of the page or the interests of the user. The content of an actively visited website could match that of your ad. Thus the ad was displayed on the appropriate page of that site.
To check whether the surge in activity was caused by adding networks:
You can ban impressions on certain websites if you believe that they don't serve your purposes. To do this:
You can also completely disable ad displays in the ad networks in the Campaign settings.
By banning ad displays in the ad networks, you lose a considerable part of the audience which may be interested in your ad.
The system selects additional relevant phrases to increase your target audience coverage. Because additional phrases are not added at a regular pace, you can expect to see some spikes in activity when they are.
To check whether the surge in activity was caused by additional relevant phrases, go to the Phrases by days tab on the statistics page.
You can see which additional phrases got the most clicks over a certain period of time in the statistics report for Additional relevant phrases. You can order the report by clicking Order reports on the My campaigns page, or on the page of the campaign in question. The report is in Excel format.
You can set restrictions on spending for additional relevant phrases or disable them completely in the Campaign settings.
Go to the Phrases by days section in statistics to identify the keywords which got the most clicks. If some particular keywords stick out in the statistics, it could be caused by external factors (time of year, a TV program, news etc.). You can also use Metrica to view specific search queries which led to your website.