In my view, because time as we conceive of it is not the same as the manner in which God conceives of it, it only appears to be predestination because of our strictly linear view. God knew that she would submit her will when told that she had been chosen to carry the son of God, so he went ahead and gave her the fullness of grace in vitro to prepare her womb for His only son.
To help explain, let me use the example of confession and sin.
You have sinned. God knows the future and knows that you are going to go to confession, so you already know that you are going to receive the absolution. So do you still have free will in this instance, or were you predestined to go to confession and therefore by extension were predestined to sin? The answer to all of this is “no.”
God knows you are going to confession, but you still have to make the conscious effort to go. You are still granted this choice. In the eyes of the Lord, before you’ve even sinned, you have already gone to confession and received absolution. But in the world in which we live and the manner through which we perceive time, the sin must precede the confession must precede the absolution. Therefore, we are answerable to our sequential understanding of time because we cannot fathom time as God perceives it. Quid pro quo, we must therefore still go to confession and we must make the decision to do it.
The same applies to the Immaculate Conception. Before Mary was ever born, God knew she would submit her will. But in the same manner that time is perceived differently, Mary was still given the choice to submit or the choice to deny. Only with this understanding of how time works differently for God and for man can one understand that the Immaculate Conception was not predestination, it was in fact in answer to Mary’s submission to God’s will.