Someone else might know the answer. But some programs do (with or without notification on the program installer) also install junkware. This might be toolbars or something else. When that happens, often the default browser homepage is changed also.

Why is a good question. Sometimes the people who develop the main program receive kickback payments if they include junkware with their program. I refer to such extras as junkware. Adobe got into strife with its users when it included Growl within their CS5 product installer range. Growl Installation with Adobe CS5 « Adobe Installation and Licensing. However they quickly responded on how to remove it due to customer backlash.
The programs that choose to include junkware with their program installer, sometimes notify in the installer these extras will be installed, but not always, such as in Adobe's case. Sometimes it is the default that these junkware will be installed. We pressured FrostWire into at least not having their junkware set as default for installation. We complained they even had junkware with their 4.20 & 4.21 installers. File_Girl produced junkware free versions instead for this forum lol.