Google’s new documentation lists three reasons why Discover traffic changes:1. Changing interests2. Content types3. Updates to Google Search
Changing Interests: Interests In Topics
Google's updated guidelines mention that declining interest in a topic can lead to fewer articles and reduced publisher traffic. Topics vary from evergreen to seasonal.
Content Types
Google says that the types of content shown in Discover are subject to change. As with regular search, the changes are meant to surface the content people are interested in.
Updates to Google Search
Google's algorithm updates impact Google Discover traffic, with a focus on signals of usefulness and helpfulness to determine surfaced content.