High SERP volatility has recorded on June 30th, and several webmasters have reported mass fluctuations in search results. The last confirmed update from Google was the second part of the anti-spam update that was rolled out on June 28th.
Considering the proximity of the recent Google update and the SERP tools starting to pick up signals from June 28th with a spike on June 30th, these fluctuations are likely due to the delayed changes from the anti-spam update.
Note: Google has announced rolling out July 2021 core update on July 1st. Considering the proximity of this another Google update, the SERP fluctuations seen on June 30th are yet unknown. If your website has been negatively hit by the fluctuations on June 30th, you’ll need to wait until the core update rolls out completely, as there is a possible chance of these changes being reversed during the core update.