I think thats because in order to roll with the c stick, you must be shielding, and if you input C stick left right up down etc on the last frame of an attack or roll or something, it inputs c stick action on that next frame(same way all actions work in melee).and I guess since the shield wasnt already up smash attacks had priority or something over the shield action.
You can test this out my simply pausing the game during your standing animation. While the game is paused just press down R, and hold the C stick any direction. When u unpause you get that smash attack. Seems if u are a frame late...lol you get jump, roll, or step dodge.
cool **** bro, I didnt actually know that b4