Goto the URL opera://extensions. Do you have any extensions in there that are not supposed to be there or any extensions that are not from https://chrome.google.com/webstore/category/extensions or https://addons.opera.com/. If you do find something odd there, turn developer mode on the opera://extensions page and click details for the malicious extension. Then, post the ID and name of the extension here.
Quite a few users are getting infected with some malware that force-installs an extension in Opera that hijacks your searches etc. and sometimes it causes Opera to close and restart. I'd scan your system with the free version of Malwarebytes to make sure snuck on your computer.