Have a look in local routing table. With iproute2 tools installed do ip route show table local. As you can see, all packets destinated to your local IPs would never go thru NICs since they are marked as local.
To force packets go via ethernet card remove the appropriate route (i.e. ip route delete 192.168.122.1 dev eth0 table local). To restore this route just set the interface down and up: the kernel would do the work to insert these routes.
