<p>Not on a whim. The adcoms are basically arguing among themselves and voting on whom to take and whom to reject or defer. They are filling in the last few slots, and it's often tough to choose between so many good applicants. One may be on the admit pile for a while but then get displaced by another when the pending files get reconsidered. Or some adcom may really like someone on the reject pile and refuse to let them go and reopens the argument and succeeds in getting them admitted. It happens this way at pretty much every selective school.</p>
<p>They have a very limited number they can accept. Once they have hit this number, every time they add a new one to the admit stack it means somebody already on there has to come off.</p>