This can happen if adware is installed without your knowledge.
1. Use free AdwareMedic to remove adware
http://www.adwaremedic.com/index.php
Install , open, and run it by clicking “Scan for Adware” button to remove adware.
Once done, quit AdwareMedic.
or
Remove the adware manually by following the “HowTo” from Apple.
http://support.apple.com/en-us/HT203987
2. Safari > Preferences > Extensions
Turn those off and relaunch Safari.
Turn those on one by one and test.
Note:
If you don’t have another browser installed:
Shut down the computer.
Press the power button. Immediately after you hear the startup sound, press and hold the Shift key.
The Shift key should be pressed as soon as possible after startup,but not before the startup sound.
Release the Shift key when you see the Apple logo on the screen. Startup will take longer than usual.
Login to your account. Launch Safari.
Follow the steps posted above to remove the adware.
Once done, quit Safari and restart the computer from in the menu bar.
* Source: http://support.apple.com/kb/PH18760