It may be happening on various levels. Process in this order:

1. Deactivate all plugins which may make this redirection.
2. .htaccess file. Look into that file and search for redirection rules with client domain. Different plugins could enter here own rules and they will be active even after plugin deactivation.
3. Check database for containing old domain. Especially table and rows wp_options.siteurl and wp_options.home. Search and replace if they contain wrong domain.
4. If problem is still here, look for javascript redirection in theme files.