Your computer might be affected by malware. It's difficult to find out what's causing this without knowing more details. Make sure that you have antivirus/antimalware software updated and running.
The most common cause for web-site redirection is spywear/adwear or more vicious malware guiding your computer to unwanted site. A good free program to prevent this is called Spybot. and
It could either be because of the execution of certain dynamic script/code on the website or because of spyware/malware or adware in your system. Make sure you have a good anti-virus and a spyware detector installed. Get AVG anti-virus, and both Spyware Doctor and Ad-aware which are one of the best anti-spyware programs out there.
This is a symptom of "ad-ware", a form of malicious software that is hijacking your computer to take you where the virus' programmer wants it to go. Do virus scan.