There could be many reasons and you may want to read some of the discussions at this thread (http://www.flyertalk.com/forum/showt...530074&page=2). One of many potential reasons could be an AC employee travelling on official business or operational reasons. That AC employee will be entitled to travel in business class and will have super priority over fare paying passengers, including one holding a full J tickets. Since you were travelling on a City Pass and did not really paid a full J fare, therefore, I surmise that your situation would have been similar to mine who had paid M for the sole purpose of obtaining an upgrade confirmation at reservation, and it follows, as I borrow from another post, “…..regardless of at what point your upgrade was confirmed or how, you were still using an upgrade. It is not the same as holding a full J ticket, because a full J ticket entitles you to exactly that - J class, without using any upgrade instruments. So, apples and oranges.” City Pass is a form of upgrade. Business class space is never guaranteed and is subject to availability at a specified window on a first come first get basis although coming from different buckets than SWU/SSWU upgrade.

There could be other reasons; this is just one of them.

*Gold is not equal to SE. If you were an SE, AC might have picked another standby passenger to offload.