you have to look at the type of department, and how they pay (or don't pay) their students. in engineering or the hard sciences decisions trickle out b/c getting accepted normally depends on whether or not a professor is willing to make a financial offer, and those decisions are made individually by the professor and pay little attention to a departmental timeline.
if you're in planetary science, more than likely individual professors are waiting to see what grants come through before they decide if they're even pursuing a new student, and then are recruiting individual students. point being, if a place has sent out some rejections, but you have not yet been rejected, the likely reason is that you are on some professor's short list, but they either prefer another student or do not yet know if they have money to make you an offer. the grant cycle is not the same as the student application cycle though, so these types of decisions often get dragged out to the very end of the application period.