1: Norn are known for being impulsive and taking risks that members of other races probably wouldn't in the name of building their legend. Thus, if a storyline involves a character getting in over their head on something, there is a good chance that character will be a norn.
2: Rightly or wrongly, bad things happening to female characters generates a stronger emotional reaction in most people than bad things happening to male characters, so to maximise drama, tragedy should involve a female character.
2.1: Apparent circumvention of the above can if a male character has a strong emotional connection to a female character. However, this is not really a circumvention, as in that case drama can still be harvested through the female character's loss at the male character's death - arguably, following the idea that it's worse to be the one that's left behind, this is still essentially a bad thing happening to a female character.
2.1 examples: Snaff and Zojja,
2.2: Norn are probably the least suitable race for leveraging 2.1, since norn in general and female norn in particular are more independent and individualistic than other races, and norn culture is more inclined towards responding to death by celebrating the legend of the deceased and getting back on to building their own than through heart-wrenching grieving scenes.
Put them together, and a disproportionate number of poor female norn is the result.