This confirms you are hitting a software bug, Juniper calls these PR (problem reports). You can try to upgrade to the newest release as there is r22 posted now in the download sites to see if this corrects the issue. If you read the release notes for this version you can see if any of the listed corrected PR match circumstances on your network.
Or you can open an official JTAC case now with the tech support and core dump information. JTAC should be able to tell from these if you are hitting an existing PR or have discovered a new one.