My personal tale on this is that given that the brain contains chaotic circuits (i.e. circuits in which tiny perturbations lead to cascading effects), and these circuits are complex and sensitive enough, the brain may be inherently unpredictable due to quantum fluctuations causing non-negligible effects.

I don’t know if the above is the case, but if there’s anything like free will out there, I’m inclined to believe that its origins lie in something like that.