The Xbox 360 port forward guide says to open up port 53 (UDP and TCP)...
http://support.xbox.com/en-US/xbox-360/networking/network-ports-used-xbox-live
When I do this, all DNS requests on my LAN stop working. Since DNS is also on port 53, my *guess* is that when devices on my LAN do a DNS request to the WRT1900AC, the router is port forwarding the request to my Xbox instead on my provider's DNS on the WAN side. If that is what is happening, that seems like a bug. Port forwarding should only occur on the WAN side of the router and not within my LAN.
Anyway, for now, I removed the port forward on 53 and things seems back to normal. Just passing along in case anyone else hits this.