I think the most interesting mystery about UFOs is how for decades the government tried to discourage reporting about them, to the point that pilots who saw things they could not explain would avoid reporting them because it might damage their careers, but in the last few years that has turned around and now the government wants reports.

The best explanation I can think of for that is that what people were seeing in the past decades was something the government was involved in and wanted to keep secret (stealth aircraft experiments, for example). Occasionally there would be a slip up and the public would see. By discouraging reports and trying to foster an environment where people who talked about their sightings would be seen as crackpots, the government hoped that people wouldn't be able to combine data sufficiently to learn too much.

So what changed? Let's suppose it is some kind of new aircraft. My guess is that someone else is now secretly testing similar aircraft, and the government does not know who. So now they want reports so they can try to figure out who it is.