susceptible to 'easier' magnetisation due to the materials used to make the hairspring. Just my guess.
Perhaps the newer movements, with different alloys and without iron content for the hairspring , it could be of better resistance to being easily magnetised.
This is a very interesting topic to investigate further. Hopefully one of the manufacturers could share with us....or Bernard could have captured some details during his IWC factory visit.
PS : Also keen to find out if the manufacturers also test their watches with induction cooker .