Cold rain, snow, wet brush/tall grass when cold, wind with above, a combo of all. At high altitudes (Cascades, Rockies) this often happens, although the soggy brush/tall grass to waist height or higher is more characteristic of the Cascades. Even overnight dew in fair weather is enough to make them soggy!
If it's warm, I just get wet--nylon supplex and its relatives dry really fast.
I've met enough cold, wet, windy conditions in the past that I never go without my rain pants. In warmer climates, I might feel differently.