Term foxing may have come from the color being similar to a
fox fur. Yet it is not agreed where the term came from.
I believe foxing can have a number of causes. I think foxing caused by fungi has a mottled appearance like
this. High humidity is definately a factor to avoid as Jeff says.
I think foxing can also be caused by the
acidification process, which looks like a more solid/consistant darkening. This is my take on foxing caused by acidification:
To slow down Foxing on folds/edges - avoid poster being exposured to sunlight and high humidity. (Moisture in the paper speeds up the acidification.) Fluctuations in temperature and humidity (as Chris mentions) increase the process of acidification. Edges are more prone to all these issues as the are more likely to be exposed to the atmosphere, than the middle of the poster.