Many legitimate programs that users intentionally install come with unexpected add-ons to browsers--you might say hitchhikers--if you're not careful when clicking to advance each step of the installation. A lot of people click "next, next, next", etc. during the installation. However, upon the next launch of the browser they might be surprised to see one or more toolbars added to the top of the page. They may also find their home page changed. Usually the toolbars/home page change are listed as part of the installation but overlooked. Others are less obvious and often have to be uncovered via "custom install" or "options". Still others might even be "free trials". Personally I haven't seen any free trials that I have ever wanted to install. I always prefer to call the shots.
Many of these hitchhikers are harmless but annoying when one has to remove them so take an extra few moments during installations to be sure that you're installing only what you want to install.