I wouldn't rule out a mass panic-type event. A similar event which closed London's Gatwick airport (one of Europe's busiest) garnered no evidence and no serious lines of enquiry. https://www.theguardian.com/uk-news/2020/dec/01/the-mystery-... See also the recent closure of Heathrow Terminal 4 https://www.bbc.co.uk/news/live/c5ypl5grg24t
There's footage of this incident, footage of monday's Copenhagen incident, and there were arrests in Oslo on monday for another incident over Oslo.
There were also unconfirmed sightings of drones around the airport in Oslo. The director of police states that "It is still unclear what was observed. There are conflicting interpretations of the observations that were made".