My guess (and not an educated one):

Marriott goofed up the rollover nights and didn't take into account that a silver with 43 nights in 2009 would roll forward 33 nights, which would post in January 2010, thus giving them 76 nights in the 12 month period. I think they got surprised by this, and it caused them to re-evaluate the rolling 12 month qualification.

A lot has been made of Marriott re-qualifying people that fell short, but I suspect that was calculated and well planned (to keep elite levels where they wanted them).
However, I feel like they missed the fact that the rollover nights would create a lot of new Platinums by virtue of the rolling 12 month qualification, and as of the date that the rollover nights posted, they have more Platinums than they wanted.