No, I don't think it's a pattern [as long as your au-pairs don't start dying ;)] Things happen to all of us daily, we just do not necessarily pay attention to them until some other factor unifies them in a strange way (think raining right after you wash your car -- don't remember the days when it did not rain and your car was clean, right?).
Anyway, just wanted to say that I'm really happy for you finding Sabrina. WHat a relief! I hope she will stay with you for her whole year and won't cause any heart (and head) ache like the other two.