From what I remember not every school sends out secondaries...and those that do send them out often do so on an interest basis.
A school will send you a secondary right away if they feel your application is a strong one and possibly hold off sending you a secondary if you are in the middle.
This way they get the secondaries back from their top prospects earlier, interview them earlier, and fill the bulk of their acceptances as soon as possible.
Some schools, like NYCOM...unless something has changed...doesnt send a secondary at all...you fill it out on-site.
