If someone has something to divulge, divulge it. Catch the media off guard and it has a better chance of going viral! The other concern, of course, is that the folks who have something to say might take a permanent vacation if it's known ahead of time that their information might be damaging.
“when someone announces ahead of time that something sensational will be announced”
Assange didn’t announce anything.
An MSNBC reporter tweeted an announcement and some people took it too seriously.