From Creatures Wikia
It is interesting to note that slink originally intended for the later C1 norn breeds (Ron Norns and the like) to be overcome with a necromould later in life. However it seems that this was discarded in favour of a simple automatic-death gene due to import shock-related fatalities.
The only way around import shock is to back up all .exp files, possibly by using the "read-only" function. This will create an error dialogue, but not crash the game.
Import shock is probably related to the as-yet un-named condition where after a crash, reopening the world would kill about half the norns in the world, seemingly at random.