Yep, I got them too and they were from Google Adsense.
A bit of research (not hard, everyone is complaining) and it turns out that I had 'ad experiments' set to On by default. Have now turned off. Will possibly take a couple of hours to clear.
Google has jumped the shark. The Google of 2000~2010 would never have pulled that shocking UX stunt on publishers 'by default' - they have crossed the rubicon and are now fully-formed cynical corporate jerks the same as any other.
I just came across this email in my ski webmaster email that was received on the 12th at 7pm.
Auto optimise is fully enabled in your account. This lets Google run experiments on your traffic to find ways to improve your revenue or user experience.
Good news. The recent experiments below showed positive results and we have applied these changes to your account. There is no action required from you. If you would like to view the experiment results, click on the experiment link under "Revenue uplift".
So the pop-unders must have been running in low numbers as an experiment and then turned on to 100% on the 12th.
One day later, complaints are rolling in and I immediately deactivate.
But I think there is a bigger story here;
I reckon Google has flat-lining growth, saturation has been reached and business everywhere has tightened the marketing belt due to Covid - So HQ has tasked all the P&L business units to 'find ways'. According to the graph chart in the email, the experiment was auto activated because it supposedly lifted revenue by 187% which is a massive, massive lift in ad-world terms. Undoubtedly this is simply because the offensive pop-under interstitial ads would have delivered huge accidental clicks.
Everything about this decision by Google leads me to think that folk are scrambling to make targets and integrity gets thrown out the window in favor of 'Hail-Mary passes'.