Default search engine keeps changing to bing

Test/Check if you have same issue in safe mode.

How to use safe mode on your Mac - Apple Support (Takes longer to boot compared to normal boot)

Note: Safe mode prevents your Mac from loading certain software as it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which are automatically created again as needed.

Check in Safari Preferences >> Search and choose whatever Other Search engine listed. Quit Safari and Test. If it has changed back to Bing >>

Suggest downloading from a respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

There are Malware-viruses and Adware-viruses that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application.

i would say that there might be an add-on or extension doing this. You can check system preferences or safari's preferences for this.