When you create a Google account in Internet Explorer, there's a weird option: "Set Google as my default homepage" and it's enabled by default. After filling out the form and clicking "Next step", you should see a browser dialog that asks if you really want a new homepage.
It's a strange option to add in a "sign up" form and it has nothing to do with creating a Google account. Google already prompts you to set Google.com as your homepage when you visit Google.com in IE and it even shows ads that send you to this landing page.
It reminds me of the Google Earth download page that has an option to download Chrome and it's enabled by default. "Make Google Chrome my default browser" is also enabled, so if you click "Agree and download" without noticing the checkboxes, you get a new browser, not just Google Earth.