SERP volatility describes how much search rankings are moving on a given day, independent of any specific site's actions — driven by algorithm testing, ranking experiments, and ordinary variance in how results are served. A single day of high volatility affecting a keyword is not, by itself, evidence of a targeted change; distinguishing volatility from a real, durable ranking shift requires watching a trend over multiple observations rather than reacting to one.
See How to Track Keyword Rankings Without Checking Manually Every Day for the full picture.