Broken Mainspring Question

[1904 Submariner]

I have an old meters first 5513 Submariner. My job is a bit hard on watches, so it only gets worn on weekends. Friday afternoon I woke up from a nap and found that my watch had stopped. I got in the car and drove straight to the AD. They took it and said they'd call in a few days. Today they called and said the mainspring was broken, but everything else was fine.

In January 2010 my watch was serviced by this AD. They give a one year warranty on parts and labor, so the warranty expired 8 months ago. I didn't ask if it was included in the service, but after reading up on this site today it looks like replacing the mainspring is pretty standard in services (especially at an AD).

So my question is: Why did my mainspring break?

The woman I spoke to on the phone suggested it might be from shaking the watch instead of winding the crown to start it from a stop. Since I don't wear the watch every day, I wind it once a week (not shake it). Should I buy a watch winder to keep the watch constantly running, or is there something else going on here that I'm missing?