Why did my quality score change in Google AdWords?

Your quality score in Google AdWords can change if you add or change the ads associated with keywords, alter the destination URL associated with your keywords, or experience a change in keyword performance, such as getting more or less clicks.

A quality score in Google AdWords is the score given to your keywords based on how relevant your keywords are to your ads and your website landing page. So if you alter your ads or website landing page (destination URL) in any way, sometimes the quality score for your keywords will change. If you create ads that are more relevant to your keyword choice, or improve your website landing page through proper search engine optimization, your keyword quality score can improve.

Another factor that can change a keyword quality score is keyword performance. If a keyword begins to perform quite well, as evidenced by your receiving a high click-through rate for associated ads, your quality score can improve. On the flip side, poor keyword performance can lower your quality score.

See: What makes people instantly click Google Ads now?