Mid-April mass rejections is my experience from last year, and I assume it will be the same this year as admissions is catching up quickly. I assume it is because up until a certain point they are not sure where the line is drawn from order or merit. Once they get the classes almost finalized they can begin to be confident that a rejection will be a rejection, because the class is closer to being filled. Personally, I would be surprised if mass rejections went out later than 20 April. Last year it was 15 Apr 21.
It already happens every year, albeit rarely, where they give out a rejection and then later have to overturn it and send either a full offer of direct appointment or a USAFAPS/FFS offer, so I can start to imagine how much more often this would happen if they started predicting who wouldn't make the cut in March. I assume they delay it this long to minimize how often that situation occurs.