She was careless and didnt have her shield active. That is also the stated reason she lost her arm. Or it could be Bungie's salute to women.
However there is another explanation it could have been the excess radiation from the nearby Covenant glassing shorted out her shields, as she may have been exposed to the radiation longer than the other members of Noble Team, thus causing her shields to be at lower strength, hence the fatal needler spike through the helmet.
ok let's get some actual facts. kat lost her arm in the live action trailer by getting injured by a banshee. the reason kat died was that, yes, she was careless and left her shields off. a spartans shield gets deactivated automatically if the helmet is removed = headshot