We lost luggage and that put a delay in the schedule. Anything can throw you off bad enough to not make a 10:35 flight. You are supposed to be at the airport two hours early, which means leaving the ship right at 8:00 - sometimes they aren't even disembarking then.
If there has been an accident on the road, if your luggage isn't right there and you need to look for it, if the ship doesn't clear customs quickly, if - for some reason - customs decides to pull you aside reentering the country, if there is an extra long line at security - you will miss your plane.