This is honestly just a wild guess, so take it with a grain of salt, but many institutions (esp T10 schools) released their decisions last week or the week before. Most institutions require students to accept their offers within 2–3 weeks. Maybe because vandy's offer rate and waitlist movement have been super unpredictable in recent years, they’re trying to see how many students decline their offers before deciding how many more to accept? Someone else also mentioned that they’re planning to accept more people this year, so by calculating how many students are choosing other schools, they might get a better idea of how big the third wave will be.