Probably the port forwarding was automatically set up by UPnP, which is also something that can't be done on a vpn without port forwarding. If you have a tracker, the torrent might also work, but then the tracker itself would have to be port forwarded.

Were you using a VPN at the time? I was under the impression that the issue is at standard anything involving ports just doesn't work with a VPN because it would be trying to get to the port on the VPN, which the VPN would rightly refuse unless you'd set it up to forward that port