In the docs - it describes a Stale workflow as:
STALE: Progression of the workflow instance requires that a background job executes, however the job cannot be found in the system. This situation can occur when an error occurs when executing the workflow.
http://docs.adobe.com/docs/en/cq/5-6-1/workflows/wf-admin.html
Are your workflows referencing jobs that are not present on the server?