The findbugs site generated in http://download.jboss.org/jbosstools/updates/requirements/findbugs/1.3.9/ has troubles to be consumed with Tycho (see https://bugs.eclipse.org/bugs/show_bug.cgi?id=380438 ). This seems to be because it is a PARTIAL IU. Referring to the Javadoc, we can read:

A property key (value <code>"org.eclipse.equinox.p2.partial.iu"</code>) for a

• boolean property indicating the IU is generated from incomplete information and
• should be replaced by the complete IU if available.

Looking at the content.jar, we can actually see this flag set to true.
It is possible that the site was not correctly generated.