Unwanted Default Search Engine in Safari, Chrome, Firefox on Mac
In your regular use of your favorite browser on your Mac, you may have suddenly discovered your default search engine has been changed to a different one. This can happen regardless of the browser you use and may affect Mac's own Safari, as well as Firefox and Chrome.
The most common cause for a sudden and unauthorized switch in default search engine inside your browser is a type of a potentially unwanted application called a browser hijacker. Browser hijackers have the ability to make quiet changes to your browser's settings, including the browser's home page and default search engine.
This sort of behavior and the fact that all of this usually happens without the user's knowledge or consent classifies this sort of software as potentially unwanted apps.
Browser hijackers can sometimes set up persistence, using different methods on different operating systems, which ensure that even if the user changes their homepage or default search engine back to their usual one, next time the browser boots up, the hijacker implants its own settings once again, without asking permission.
It's easy to see how this sort of behavior can be disruptive to the user's browsing experience. This is exactly why similar apps are classified as "potentially unwanted".
The most common manifestation of browser hijackers are browser extensions that install quietly. If you find that your favorite browser's home page or default search engine have been changed without your knowledge, the first place you should look to remedy the situation is your browser's extensions menu.
In a lot of cases, simply removing the extension you don't recognize, which acts as a browser hijacker, will allow you to restore your default search engine or home page permanently. In other cases, the issues may be more deeply rooted and require a case-by-case approach for their removal.
Browser hijackers that simply change your default search engine may have more unpleasant "features", such as logging your IP and your search queries, then selling them to third-parti entities for profit. Just because potentially unwanted applications don't have the same elevated threat level as full-blown malware doesn't mean that they are harmless or should be kept around on your system.
Of course, if you keep an updated and fully-featured anti-malware application on your system, the chances of a browser hijacker ending up on your system in the first place will be significantly lower.