An outcast in WOWFC refers to any member of a side or faction that leaves it deliberatly on purpose. Certain reason can come from this and it can be any form of leaving/ quitting a side.
When Fury left the Alliance, he did so by leaving and killing off a few soldiers. This took him as an Outcast, on the level that he is not a threat to worry about by the Alliance, but that if found should be killed or brought back to the city to investigate.
Dagamier became an outcast of the Alliance by leaving off to become a neutral warlock. He killed many Alliance and almost killed Kelsisco. He was seen as a extreme threat to the Alliance, and was sought out to be killed or brought back to Stormwind (mainly the job of Kelsisco).