They don't actually fail like an icicle - ice is brittle and shatters by fast crack growth, carbon-fibre reinforced plastics (to give their full name) are fairly resistant to crack growth. The issue with carbon forks is that a metal part will likely give some indication that it's going to fail, often in the form of a visible crack or some sort of ductile deformation, while potential failures in carbon components aren't always as easy to spot.