If you live in an area where it doesn't rain much, a little water is likely to make your roads very slippery. I don't know what it is, but roads are slick when the rain first starts. After they're thoroughly wet traction improves again. I don't blame my tires; I ride cautiously.
It's because the water interacts with all the car oil and junk that gets trapped in the pavement when it's dry, and turns the road into an oil slick. With enough moisture, the oil and dirt wash away, which is why traction improves, but light moisture just brings it all to the surface.