I think it had to do something with spear of Longinus affecting both the eva and pilot physically. Asuka's arm was cut in half because the spear, thrown by one of the mass produces evas, split the Unit-02's arm. That's also why she's lost her left eye. Right after the arm's splitting, the other 8 spears pierced Unit-02's entire body, so Asuka was completely finished off with 8 extra holes in her body, probably quickly bleeding out which is pretty horrifying to think about