Why does source usage change?
Updated
Short answer
Engines that retrieve live pages at answer time cite different sources as the web changes. Movement across a week is meaningful; a single day's difference usually is not.
Normal movement
New articles are published, old ones are updated, and engines sample different pages from one run to the next. Expect the long tail of your source list to shift constantly.
Movement worth investigating
- A page that was cited consistently for weeks disappears.
- A new page appears across several engines at once.
- The change lines up with a change in your visibility or sentiment.