User blog comment:VedranTheII/Pretentious Diaries 2: What tricks would you teach your Aura?/@comment-5217560-20150719083609/@comment-4010415-20150719164240

There is the fact that the eruption occurred after the shot Glynda blocked splattered onto the ground around her, so it's possible that Cinder still had control of that projectile after it landed on the ground and was able to pull it up into a powerful eruption.