handbrake (or specifically, the ghb front end) does not honour the GNOME network proxy settings, nor the http_proxy environment variable. In addition, it runs in one thread and dials home on startup, using synchronous calls, so in an environment where outbound port 80 packets are dropped, it freezes up:

ghb       19841    jon   10u  IPv4  95315      0t0  TCP> (SYN_SENT)

It seems to be trying to talk to handbrake.fr, and based on a glance at the source code, is either checking for updates, or fetching '/appcast.xml' which seems to be a blog about the app.