So? Biotics are part of her, they could work automatically as a defence mechanism even if she was afraid.
Alright, let us take a different approach.
Let me set the stage: A long-time friend of yours suddenly pulls out a weapon, with the intent to kill you, shoving you to the ground in the process.
Let's assume you are armed, and discard any delays it would take for you to pull out your own weapon.
Would you be able to react in an instant, or would you take precious seconds to realize what is going on and overcome the shock?
Biotics require conscious usage of "muscles" that are the Eezo nodules in your body. Take that in account.