I need to determine what drives the Status field.

Kristen, 

At risk: 

There's a 10% tolerance between Expected Progress (a linear calculation of work/duration of a task) and actual %complete. 

http://usermanual.clarizen.com/#scheduling-status

Off track: 

Passed Due date - calculated by a nightly job (midnight).

If not At Risk or Off Track, then work item will show On Track so long as it is Active, not Active will not show status.

Sincerely, 

Boris Krutiy

Also, you can manually change the Status by double clicking on the column in your view (need to show column in your Work Plan view to do this).