On Swiss, like most European airlines, the business class section is not fixed like it is in the US/Canada. It is just standard economy seating with better service and an empty middle seat. So they can play with the size of the business class section on any given flight just by adjusting where the curtain is. If they're selling more business class seats the curtain moves back -- if they're selling fewer, it may move forward. If you have booked a preferred seat in economy they will try to keep your seat consistent (if you've booked the second row behind business class then you'll move with the curtain. I've think that I usually only get one email telling me my seat has changed in these situations -- but to be honest, it happens so often that I may unconsciously delete them and move on.