SharePoint Workflow Manager workflows allows creating workflows that communicate with the SharePoint objects through the REST API calls. End of the day a REST API is connected through the Client.svc endpoint of a particular site. Somehow there are situations I have experienced Cancellation of workflows due to the failures of REST calls.
There are instances where when you make too many REST API calls, a 429 HTTP Status Code is returned. I have explained this in one of my previous articles here. That is a carefully handled scenario of the Client.svc.