Choice Privileges - How does Choice decide the points expiration date?

I have some points in my account and originally was to expire by the end of 2008. However, the expiration date got changed to much longer after I used the Choice creditcard to purchase over $1500 goods.

Anybody know why this happened and how Choice decides the expiration date of the points?

See here: http://www.flyertalk.com/forum/choice-privileges/898499-do-cp-points-expire.html

Your new activity shouldn't have changed the expiration date for existing points. Accounts have three point expiration dates.