Check your extensions for Shop at Home Special Rebates or similar. Also check your Add/Remove Programs. It’s one of those sneaky ad extensions that can get on your computer without you realizing it. If it’s anything similar to other sneaky ad extensions, it was stuffed in with some other installation of a major product as a little ticked-by-default box on some page you didn’t bother to read.
Kabam and ShopatHome.com are business partners. Kabam has also partnered with Kongregate for some of the MMO players who are not playing on Facebook. There are threads on Kabam’s forums complaining about similar redirects and stealth installs of ShopatHome.com extensions.
EDIT: I have no advice for avoiding getting the toolbar in the first place. I’d start by editing my Hosts file to send kabam.com and shopathome.com to 127.0.01, but that may be a bit extreme. You can also try avoiding Kabam games on Facebook. They ARE the plague, after all.