Yes they can and do happen, hurricanes at Disney. Last month Hurricane Matthew threatened to hit Disneyworld but fortunately, it did not hit as badly as anticipated. Disney closed at 5 PM on Thursday 10/6 in anticipation of Matthew (as did all the other theme parks and recreations in the area). They also were closed for Friday 10/7. This was done to protect the guests as well as the cast members.
It was amazing to watch them prep for the arrival of Matthew as they secured everything they could. They also prepped in the resorts with food, extra entertainment like character greets and games.
I suggest checking into travel insurance for your Disney trip, weather it is during hurricane season or not. Best to check with travel agents.
Remember this is only the 4th time Disney has closed the parks since 1971. It is certainly a rare occurrence but it can happen. SAFE TRAVELS!!