A (Java JMS) client disconnected from the broker. (Probably killed with a SIGTERM.) It had a queue for a topic that apparently continued to grow after the connection was closed.
We have a cron to periodically log all queues and other broker information using qpid-stat. The next log after the client shutdown does not show the queue. However, the queue was apparently still being maintained internally by the broker and had new messages appended to it. The broker eventually ran out of memory. We have the broker JVM set up to dump its heap when there is an OutOfMemoryError exception. Examining the heap dump showed most of the memory was being used by the queue from the client that had disconnected.