It could be something like an adware/browser hijacker. It's usually nothing to worry about but as you might have already noticed, it can be a little bit annoying.
If you are indeed infected by one, then you should be able to easily remove it by using an Anti-Malware. I personally use MalwareFox. It has a free trial so you really have nothing to lose.
If it doesn't detect anything, then chances are that you or one of your programs messed around with the Chrome startup/home settings which is something that you can easily fix from the settings tab.