I noticed a spike (or drop) in clicks (or expenditures)

The number of clicks could have changed sharply for the following reasons:

You made changes to your ads

An abrupt spike or dip in activity could be caused by any changes you recently made to your ads.

Even minor changes in ad texts and campaign settings (time, impression region, strategy), as well as changes in the budget 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.

Ad displays on content websites have become more frequent

You ads will be shown on sites of the Yandex Advertising Network if the ad's theme corresponds to the site's theme or the user's interests. 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.

A spike in appearances on content websites can only be seen in the Professional interface of Yandex.Direct. To switch to the Professional interface, open the My ads page and click on Go to Professional interface at the bottom of the page. Make sure that Professional is selected in the window that appears and click Start using the service.

To find out the number of impressions and clicks on content sites, open the statistics of your campaign, select context in the additional settings.

New words have been automatically to the keyword list

Yandex.Direct automatically adds negative keywords and plus words to those keywords, for which, collected statistics indicate, a large number of appearances leads to insufficient clicks. Refined phrases are marked by a special symbol. Added negative keywords are highlighted in gray.

More information about this function can be found in the Help section.

The Statistics was affected by external factors

The average number of keywords, for which your ad is shown, can depend on external factors (time of year, a TV program, news, etc.). To find out which phrases were clicked most, switch to the Professional interface, or use Yandex.Metrica reports.

I couldn't find an explanation for my particular situation