If you are seeing it happen on all sites, then you might have a bad browser extension or other system-level malware like a virus.
However based on your description and experience, I suspect that you are only seeing this happen on some sites, in which case, those are likely just regular popups. Some sites are designed so that as soon as you click anywhere on the page, it pops up (or under) an ad page in a new window (some do this only once per day, others do it every single time).
The ad pages can be regular ads, but many now use social-engineering and phishing to push scareware with things like warnings about outdated software (Java, Flash, etc.), virus infections, and so on. They can be quite convincing too because they specifically try to make them look like native programs or legitimate websites.
Of course these are not to be trusted and you should make sure that you are using security and anti-phishing software. Chromium (and Chrome) has a built-in anti-phishing system, so make sure it is enabled (though it may not yet include a given site).
Also make sure that your popup-blocking setting is correct and no exceptions have been added:
Figure 3: A fake Flash update page that warns you of security and performance issues with Flash and prompts you to update. Note the use of genuine looking styles and themes for the page to match that of authentic Adobe Flash pages (at least at the time). Also note how the URL of the page itself and of all of the links on the page are to somewhere other than Adobe.