Your website has probably been hacked with a conditional redirect.
The hack may not be in the website code itself, in which case you won't be able to see anything amiss there.
It's more likely that the hack is in a separate, but legitimate, file named .htaccess on the host server, in the same directory as your website code. Entries in the .htaccess file can control, among other things, basic redirect functionality. In the case of a conditional redirect hack, a malicious entry in the .htaccess can redirect traffic to another site based on the referrer (i.e., which web page the traffic is coming from). For instance, traffic may be redirected to a malicious site only if it's detected as coming from a Google Search results page.
In short, regardless of how this looks, it actually has nothing to do with Google Search results, and is not something that Google has any control over.
If you can't find, or don't have access to, the .htaccess file associated with your website, you'll need to talk with your hosting provider to help sort out the issue.