I'm pretty sure that you're infected with an adware or a browser hijacker. It's weird that your Anti-Malware programs didn't detect anything. Do also give a try with MalwareFox, it might detect something.
Other than that, do try to reboot into safe mode without networking and perform a scan again from there.
If nothing ever works, then you might just need to manually find and remove everything that causes this or maybe even reinstall your OS.
Ive had the same problem. Also the adware closed chrome every time i tried to Google "adware opening tabs"
Avast did not find any thing as well.
In AppData (CUsers/**username**/App Data) I have found folder from "Mail.ru". After its deletion it lets me Google again, and probably will stop opening the tabs as well (will have to wait for a while to be sure)
Try check the AppData folder and delete suspicious folders that were created on the day the problem started. (if you are unsure with any folder, you can Google it - for all "good" folders you should be able to find their purpose)