The issue I see is that the traffic is coming from a 192.168.1.X subnet and going to a 192.168.4.X subnet. That really doesn't make sense since you are running on a 192.168.2.X subnet.

It sounds like it is receiveing misaddressed packets from the outside connection. Can you confirm the port(s) that have been receiving those packets, your outside connection IP address and the subnet group used on your outside connection to see if the addressing corresponds?

Good catch seeing my typo.

I don't think that your router has been compromised, I would recommend trying to capture some of the questionable packets to analyze their structure to see if they all have the same contents or if the data changes. The UDP flood may be the result of an attempted denial of service attack or maybe even the result of an error on an outside connection, packet analysis can help you to identify the cause and potentially modify your firewall rules to discard those packets.