I'm surprised no one has mentioned this.
If it hasn't rained in a while or if its a busy road, there is a LOT of oil, tranny fluid, and just general grime on the road surface. When it first starts raining the water sits on top of all this, making the surface insanely slick.
However, if its a really good hard rain, all this junk gets washed away, riding after a hard down poor once the roads dry out is awesome, its like a freshly cleaned track.