There are a number of things which can cause your wrought iron fence to rust:
Chipped paint. Any time the paint on your fence is chipped to expose the iron underneath, rust is likely to develop in that spot.
Weather. Rainstorms and high humidity can both cause rust. Wrought iron fences are more likely to rust in highly humid areas.
Dirt and plant growth over the fence.
In addition to the obvious red spots, corrosion can have some other effects.