I discovered yesterday that when I went to install a plugin, the "Install" button took me straight to a "dodgy website".
That probably means that your site is hacked or you are running a dodgy plugin or theme on your installation.
Can you deactivate all plugins and switch your theme to Twenty Fifteen? This is a temporary step and I want to see if you are running a plugin or theme that's doing that.
If you still have that problem then your site may be compromised and you'll need to start with these article.