When a metric moves, open an investigation from the finding or start one yourself. It splits the change into impressions, position, click-through and query mix, and tests candidate causes.
Each cause carries a confidence tier. A cause is described as confirmed only at the highest tier; below it you will read "possible", "likely" or "strong" contributor. The methodology page lists the thresholds.
Every finding has an alternative explanation on the back of its card. Read it before acting.
Limits
Investigations per month: Free 5, Starter 30, Pro 150, Agency 1000.