It may be unexpected, surprising, formerly secret, and may come from an exclusive source. Events witnessed by many people generally cannot become scoops, (e.g., a natural disaster, or the announcement at a press conference).
Something that is surprising, unexpected, and perhaps from an exclusive source can be considered a scoop. Conversely, events that are widely observed usually do not qualify as scoops, such as a major disaster or an official statement made at a press conference.