Problem: On my iMac, I'd type search keywords into the Chrome browser's address bar. Google is my default search engine. After the search results displayed on the Google page, the tab would refresh and load the search into Yahoo.
Diagnosis: Browser malware installed somewhere in Chrome, likely a new extension. Not a virus, but annoying behavior nonetheless.
Solution: Resetting Chrome to its factory default settings, which also disables all extensions. Confirmed that searches worked as expected after the reset, which confirmed the bad extension hypothesis.
Turned on each extension one at a time and ran a search to determine which one forced the search to Yahoo.
The culprit: The Facebook Video Downloader extension, also called Social Video Downloader on the Chrome extensions site. Deleted this extension and searches have worked fine since.
Reputable products: A few products were recommended for detecting or removing malware or other issues. I did not install Malwarebytes nor Combo Cleaner; they look reputable but I don't know -- I'd want to find recent reviews of them. I installed Etrecheck to run a scan of my system and it found only minor issues, nothing related to this problem. Etrecheck can repair some damage, but its main value is in providing a report on your Mac's setup you can use to paste into a support message.
Avoid: Adware Removal Pro (reference)
Other links of interest: