Google Play restricts full-screen ads at unexpected moments, ads disguised as system notifications, and ads that appear outside the context of your app (like on the home screen when the app is closed).
Very common with aggressive ad SDKs configured for maximum fill rate — interstitials on every screen transition, or out-of-app ads that appear even when the app isn't open.
"Your app displays ads outside of the app's normal UI or when the app is not actively being used, which is not permitted under the Ads policy."