This doesn’t have to be a plugin issue.
If someone puts a link on their website to your blog say for example: www.example.com but not only that they place some random string: www.example.com/?foo=bar in that link to your site then when search engines crawl this page with the link to your site on google or any other search engine will see it and try to follow the link to your website.
As is the URL is technically valid the search engine will cache it and serve this new URL to its users.
Have a read over this post at Perishable Press that talks about it in detail.