Another possibility is that they are using naive port-based filters in their traffic shaping rules, and it thinks that any encrypted-looking packets not destined to one of a white-listed set of ports is torrent or other P2P traffic.

If you run the server side of the VPN as well as the client, you can test that possibility by trying other known ports (1194 that OpenVPN usually lives on, 433 if that isn't already directed elsewhere on the target address, ...).