FlyerTalk Forums Expert How-Tos and Guides
 

Possible reasons
-- Oversold economy / lower cabin
-- Irrops of other flights creating an oversold economy situation
-- change of scheduled aircraft ( or an expected change), this is to protect against overbooking business
-- broken seat(s)
-- pilot / crew rest seat(s)

Generally, this is good news for those on the upgrade list and a possible chance of Op-ups for some.