OpenNMS is the work of a large number of people. In order to maintain some order with respect to the copyright to the code, every contributor is asked to sign a "Contributor Agreement".
The OpenNMS Contributor Agreement is based directly on the Sun (now Oracle) Contributor Agreement. It implements the concept of a dual copyright. This allows the OpenNMS code copyright to be consolidated under one organization (The OpenNMS Group) while allowing the creator of the code to maintain all of their rights.
Sun provides a rather comprehensive FAQ on this agreement.