If the log entries appear around the time after the router rebooted, then they can be explained:
If they appear long after reboot, but only sporadically, then it could be that there was some connectivity issue with the upstream DNS servers.
Most of the normal DNS traffic (initiated by normal clients, non-DoH/DoT/DoQ, also not zone transfers between DNS servers) is normally over UDP. The clients only switch to TCP when the response is too big (server answers with a flag that says that the answer has been truncated), or when they did not get a response to a query over UDP after a couple of seconds. In your case within your LAN, the flood of TCP SYN on port 53 of the router is probably due to the latter.