No, unlikely a composite component would shatter-like-glass, as it's a matrix of fibres within a substrate.
Although a lot of strain-energy would be released when a component does break, that then would release the strain throughout much of the rest of the component.
The fin was not found in a thousand pieces, nor would that be expected !
One hypothesis, is that it failed due to inertial loading, when the fuse entered the water (in line of flight - read axially in this case), decelerating at a very high rate, as it's(the fin's) supporting structure progressively buried itself under water, with the fin still in free-air (little balancing drag)
No doubt BEA have already calculated the deceleration required, in round terms, to fail the fixings through forward bending in this manner... from its mass and vertical mass centroid. I would imagine this would be many tens, maybe 20 ~50g or more (but don't shoot me unless you've done some sums as well
