Why is yahoo my search engine - anyone else seeing this?

Nine times out of ten that's a browser hijacker, usually bundled with a free download or a dodgy extension. Check your extensions first and remove anything you don't recognise, then set the default search engine back in settings. If it flips back after a restart: on Chrome look for "managed by your organization" in the menu - if you see that on a home PC, the junk installed a policy and you need to remove that program too. On a Mac check System Settings for any profile you didn't add.