Outgoing Ports with Apache/WordPress Installation

I run three ‘virtual hosts’ on an Apache webserver. One is powered by WordPress, the others not. The server is behind a router, with port 80 forwarded.
Whenever a page request is made to the WordPress site, Apache makes a TCP request to the WAN IP address of the router on port 80, from a random port (eg 1360). The request is blocked by the server firewall, but the site serves the page to the requester without problems.
No such TCP request is made by the non-Wordpress sites.
The server firewall blocks the request, because it seems illegitimate; my understanding is that webservers respond to page requests on port 80, as well as listen on it.
What is going on? Why does a WordPress installation cause Apache to make a non-standard TCP request, effectively to itself, since (if the firewall didn’t block it) it would be forwarded by the router to it’s ‘listening’ self’.
I’m puzzled. Any light anyone could shed on this would be appreciated!
With thanks.