WCCP router redirected traffic to a ProxySG on port which is set to bypass. And because the proxy is pointing the WCCP router as a gateway, the packets are sent back to WCCP router and at this point routing loop occur.
This problem can be avoided by excluding the proxy connected WCCP router interface from the WCCP redirect in setting.
In order to exclude inbound traffic on an interface from redirection. You should use this command on the router interface to which the ProxySG is connected to prevent redirection loops if you are using outbound redirection on the router. If you are using inbound redirection only, you do not need to use this command.
Router(config)#interface <interface_name> Router(config-if)#ip wccp redirect exclude in