Issue – Has your iPhone or iPad changed it’s default search engine to Yahoo? Is Safari loading the Yahoo search page on launch?
How does this happen – Touch screen accidents… The iOS Weather app has a link to Yahoo Search at the lower left and you may accidentally hit it (and Apple does $urely rejoice). Other apps may have similar links. Once you land on the Yahoo search page you may be prompted to set Yahoo as the default and if you accidentally tap Okay your default search engine is now Yahoo. Blame your pocket.
Solution – Tap Settings > Safari > Search Engine – choose your preferred default.
Apple does not currently allow any options other than Google, Yahoo or Bing. You can download apps for alternatives like Duck Duck Go.