I think I know what happened. You were going downhill and dragging your brake pretty hard (8% at only 15mph means you're braking hard). This puts a lot of braking forces into your front wheel. Then while braking you ran over the wiperblade with your front wheel, and this caused the front wheel to slip and you lowside the bike (ie, bike leaning to one side and then crash). And a wet/damp surface would make this scenario even more likely!
In addition, when you ran over the wiperblade, this caused the wiper to bounch off the ground and then your rear wheel must have partially caught the wiper and drug the wiper along for the ride, and that's why you would see the wiper at the scene of the accident. I'm almost certain that the initial impetus of the crash was not the wiperblade jamming into your wheel (front or rear) because if this were to happen, your some spokes would have been thrashed and most likely bent or broken and wheel would be way outta true.