Hello friend,

What factors affect Quality Score?

The relevance of each keyword to its ad group
Landing page quality and relevance
The relevance of your ad text

There are several reasons to affect quality score
1. un-relevancy
2. Your Keyword biddings
3. Website and Landing Page, not optimization
3. Duplicate Content
4. Low search volume Keywords
5. Non Relevance keywords target in Google Adwords

Quality Score is Google's rating of the quality and relevance of both your keywords and PPC ads. It is used to determine your cost per click (CPC) and multiplied by your maximum bid to determine your ad rank in the ad auction process. Landing page quality and relevance. The relevance of your ad text.

Expected Clickthrough Rate (CTR) Expected Clickthrough Rate (CTR) measures the likelihood that your ads will be clicked when they are shown for a specific keyword (excluding other factors such as ad position and ad extensions). ...
Ad Relevance. ...
Landing Page Experience.

What Factors Affect Your AdWords Quality Score?
Expected Clickthrough Rate (CTR) Expected Clickthrough Rate (CTR) measures the likelihood that your ads will be clicked when they are shown for a specific keyword (excluding other factors such as ad position and ad extensions). ...
Ad Relevance. ...
Landing Page Experience.

There are many reasons available for affect quality score. They are
1. CTR
2. Ad relevance
3. Duplicate Content
4. Keywords stuffing