Why Google chrome set any search engine as default?

Hi guys, I'm experiencing this problem from 3 to 4 months I don't know what is wrong with google chrome it sets any search engine as default by itself when I change it back to google search as default again when I open chrome it sets Yahoo or bing or ask.com etc randomly. Please help me what should I do to fix this problem even I I delete all search engines from settings the same problem takes place.
