Your retargeting set — people who visited the site recently — is showing your ads to the same people about 7 times per week. A week ago it was 5. Click rate dropped 38% over the same window.
In plain English: you've shown your retargeting audience the same ad until they tuned it out. You're now paying more per click for the people least likely to buy. Either widen the audience (a 90-day window) or refresh the creative — usually a single new variation drops frequency back under 2.0 within a week.