This behavior is normal. It indicates a positive engagement with your website. When someone clicks on one of your Google Ads ads and when auto-tagging is turned on, we assign a unique click ID (gclid value) to that user.
If it returns directly to the website (for example, by entering the URL or via its favorites) instead of going through a referral site, the source of those additional sessions is assigned to the last known campaign. In this case, it is a Google Ads campaign.