Several times in my life (and I'm only 42), I have sufferred with Alopecia Areata (which is what you mother seems to have). I have had test after test and there is no underlying clinical cause. If it were to be a side effect of a medication or diet it is far more likely that the hair loss would be more even across the scalp instead of being very localised.
Alopecia of any kind in women (and men for that matter) can be quite a traumatizing turn of events, and stress of suffering from it can ADD to the effects, making the hair loss worse. I can become a viscious cycle.
If it is caused by stress, it is still Alopecia Areata...the cause is irrelevant to the name. It may come back in a short space of time, or it may not. In some instances it may never return, though I hope in your mum's case it does.
In my father's case, when my mother died he lost every hair on his body, and none ever returned. He adapted to it quite quickly, but from personal experience I know it can be much more difficult for women.