I am trying to figure that out myself... I called one school and they told me that my application had been reviewed but had been put aside to be reviewed again... Shouldn't that be a waitlist?
Although a waitlist or a rejection are pretty probable in these situations, they are not guaranteed. If your application is put aside for further review, this sometimes suggests that the school in question thinks that you might be a good candidate for admissions but that you are not an auto-admit.
So, instead of rejecting you outright, the school wants to hold onto your application and reassess it near the end of the cycle when they have a fuller picture of the incoming class. At that point, Adcomms will make a decision based on what they think will allow them to construct the strongest, most diverse class possible.
Admittedly, there are some schools that are slower at processing things than others, but the first explanation is also true at many schools, so a delay need not be a bad thing (even though it can be frustrating).