From what I have heard (including at a BCNA conference), severe stress is not considered a cause but can definitely be a significant contributing factor. That said, so can other things.
Regarding exercise when you work - nothing out there seems to be available for us but it is important to schedule it in (notice that I said schedule it in not fit it in). I work, run kids around, and live miles from anything and I have really struggled over the past 3 years to find something I can access and maintain. Everything was either impossible or added it's own stress onto me when I'm trying to keep my head above water. I now leave work early twice a week (for as long as I can negotiate it). On those days I go to a local gym with a pool and swim, very badly and very slowly, for 30-40 minutes. I get to school pickup just after the bell goes. At home I have a secondhand treadmill that I use every morning for 30 minutes - I used to try to walk outside but in the summer it conflicts with taking my daughter to swimming and in the winter it is too dark. In the evening I do about 20 minutes of arm and leg weights while I'm watching the news. I may add things on as time goes by but I'm usually pretty happy keeping to a routine and not having to think about it. I give myself permission to have a day off a week or to tone things down if I'm feeling that a particular joint is a bit painful (with Letrozole I seem to injure myself very easily).
As I said, it's taken 3 years to get this happening so that I can make it work with the stuff I have to do. By doing it this way, I've also minimised the problems that can easily derail me - cancelled bookings, waiting for doctor's in cancer exercise clinic, bad weather, etc. Of course, the pool might develop a leak, the treadmill might break down, work might insist I go back to full-time hours... but at the moment, this is me.