It's that time in the application season--time to create a thread for declining offers/withdrawing applications. If you're declining an offer/withdrawing an app, post about it here.
To echo what MtnDuck said: the effect cascades. By your decision impacts hundreds of people indirectly, and at least one person directly. Your decline opens up a spot which enables someone to decline their lesser offers. If most shifting happens in April, there just isn't enough time for adcoms to go down the waitlist. If there is on average 3 day per offer turnaround, the adcomms can't get through 10 people on their waitlist in 3 days -- when the declines really happen.