According to various reports, there was a sailboat about 35 miles offshore in the launch hazard area that did not respond to NASA radio calls to it, nor to a NASA surveillance plane that flew over it an tipped its wings seeking to get the boat's attention. Reportedly, one person was observed by the NASA surveillance plane in the cockpit of the sailboat.
Since the sailboat was in international waters (more than 12 miles off the U.S. coastline) the boat was legally allowed to be there even though NASA had issued a Notice to Mariners warning all vessels of the danger during this launch and requesting that vessels stay away from the published hazard area during the 10 minute launch window.
It is quite possible that the sailboat crew was not even aware of the fact that they were interfering with the Antares launch.
NASA's responsibility for ensuring public safety during launches required that the launch be scrubbed due to the potential risk to that sailboat if the flight would have had a first-stage malfunction and then required a command destruct action resulting in a large debris field in the launch hazard area, or if the spent first stage would impact it with the defined launch hazard area.