In addition to on-page factors like layout and page topics, there are many other things that can affect your CTR, including some that can, and sometimes do change overnight.
1. Traffic source – Perhaps the most significant factor affecting CTR is your traffic source, if there was a sudden change in your traffic sources then you will likely see a big change in your CTR. For example, if you recently started to rank for a keyword that is not extremely well targeted you could see your average CTR drop.
2. Advertiser Auction – PPC programs like AdWords are auction-based so new advertisers can move into the keyword market at any time and their ad text or image may not generate as high of a CTR from your audience. Or, a longtime advertiser that gets high CTR may pull out of the auction completely, or block your website if they are not getting high enough conversions.
Due to the auction-based nature of ad platforms, it is extraordinary to have a CTR that does not fluctuate at times.