There are some people who cancel a SSRS report because its taking to long by just closing visual studio or their browser. Then they reopen SSRS/browser and try and view the report again. In the mean time the previous report they were trying to run the query is still running on the database. This takes up quite a bit of resources and people think that there query has been cancelled, but this in not the case.
This is a common problem with users accessing reports. There is a way to stop the query from running in the background by using the KILL command, however you will need a DBA to do this for you unless you have permission to do this your self.
Hope this helps other who are struggling with the same issue 🙂