These are event specific and situational. Not a general fear of flying. If you are assigned a flight to a location where there are circumstances which make you concerned for your safety, you can drop the trip.

I have seen this happen on Military flights, although voluntary, sometimes a reserve will be called and they may not feel warm and fuzzy about flying into a war zone. I can see where some people would be concerned about NRT, especially where pregnancy or suppressed immune systems are a thought or concern.