There are only a couple of posts on this across the web, none of which have any replies to them. This is because this issue relates to any number of failures that can occur with the Workflow Manager service.
One such error is when you attempt to send a DynamicValue as the request header to an HttpSend Activity. If any of the headers have a significantly large string to them, you will cause a buffer overflow in the Workflow Manager Service.
In Workflow Manager 1.0 RTM and service bus 1.0 RTM, this will lead to an error reporting service to consume the remaining memory on your application server and thereby rendering that app server useless. In the Workflow Manager 1.0 Refresh and Service Bus 1.1 versions, the error simply kills the Workflow Manager service over and and over again.