From being involved with high altitude spy planes and stealth planes during the Cold war through to thinking about the many satellites that are put into space that relates to national security and even the clandestine space plane, we have no idea what it's for...

There is simply no precedent to be upset about a lack of transparency from NASA. Some of that is national security. Some of that is to protect IP. Some of it is just because of the way governments work.

Factor in that actually it may relate to a private company's operational issue (SpaceX) there's even less reason for transparency.

I don't think we will ever know