They sometimes hijack innocent sites
either for profit or for fun. I found that once on an aviation website called Aero Trader Online, a site to buy and sell airplanes. I visited the site one evening and went to their search page. When I entered some search criteria, the page jumped to a child porn site. I immediately notified Aero Trader Online and the FBI. Aero Trader Online was unaware their site had been hacked and was very happy that I notified them. They fixed the problem right away and it hasn't happened since. I never heard from the FBI.