On a previous boat, I had a Gori prop come off (fortunately it occurred in a harbor but a diver was unable to find it). The prop was about five years old and had lots of use.
The "expert" consensus was that the shaft key had not been exactly the right size and it eventually allowed the prop to wiggle a bit on the shaft. Of course the key was lost
too so this is conjecture. I was told there should have been some tell-tale vibration before the prop fell off but I never felt any.
You might want to pull the prop and check the key's fit.