This has been going on for a lot more than 50 years.
Governments do not know what it is and governments cannot do anything about it. That last part is what they don't want people to know and the main reason for all the secrecy.
...and then [why] suddenly change?
A cynical move to distract people from economic crisis and the increasing failure of government to manage or control world affairs. The "elites" just are not as smart as they believe themselves to be.
Or maybe there has been some falling out between the Reptilian master race and their Earth-descended minions who rule over the rest of us. See the John Carpenter movie "They Live" for details. Stock up on your Hoffman lenses.
I watched a show on Smithsonian channel very recently about Hynek and how he started out working many years for the Air Force attempting to squelch, silence and humiliate people who were reporting UFO sightings, only later to get out from under the program (blue book I believe) and into the private sector where you seemed to promote the idea that a certain percentage of sightings were not explainable by nature nor known man made technology.
The excuse they were giving for why the government would work so hard to suppress reporting and general knowledge about UFOs (this is the first time I’ve heard this excuse/theory) is for national security reasons they didn’t want the soviets or any enemies faking UFO sightings as a part of a diversion distracting from an attack or invasion. This makes very little sense. What enemy had the technology to penetrate our airspace and create fake UFO sightings in the first place??? This seems really weak at best, and if it was actually true, what would it say about the level of our enemy’s technology? Wouldn’t it even suggest that they though our enemies were behind the actual signtings, and not just potential distractions of them???
The most logical reason for suppressing the sightings so aggressively for so long is that it is neither alien nor foreign.