Yanno, I got a really dumb tinfoil hat theory...
Wanda was FAQ's predictamancer.
Remember, while her primary interest
is in Croakamancy, she has aptitude in several areas of magic, including Predictamancy. In fact, she's had to deal with Finamancy/Lookamancy before, and she hated it
So if Wanda was the Predictamancer (and Banhammer would have no use at all for a Croakamancer, so she had to be doing something else at the time), and exceedingly disliked her station, then it would be entirely reasonable to assume that she would be more loyal to Fate than to her own side.