I've had that happen to me on a Valk and on other bikes.
On the Valk, I got a new front tire with release agent on it, went out in the shop parking lot and did some slow maneuvers (to set new brake pads), and even though slow, I turned too sharp, and down she went. Bike only slid about 6-8 feet (on the guards), but it pitched me off hard on my back, sliding backwards, since it went down so fast. No damage to me or the bike (just my pride).
I once did a fast hard turn though an intersection and dumped and slid more like 40-50 feet (motocrosser). I went out and looked, and there was a great big gasoline spill all over the intersection, which makes rubber tires slippery as hell. Especially big old knobby rear tires. Bike fine, bruised my hip where I hit in the fall (but no other injury; suited up in the cold).
Whatever you got into, it reacted with rubber to become super slippery. Some stuff stays on the tire longer than other stuff. It happens.
I slipped and slided and fell all the time on my dirt bikes. (it's part of the fun)
Also, the first few minutes of rain almost always makes the roads real slippery, before washed off better with more rain.
