The sound of knobs is normal. The only things I can think of is that cross tires, like MTB tires, will probably wear off quicker on asphalt and that knobby tires may provide worse traction on hard surface, due to less rubber contact with the pavement. Although I doubt it was the reason for your fall, the traction is not THAT worse. I rode on knobbies and never had problems other than noise and extra drag.

When you fell, did the front wheel skid or you just lost balance? But really, I doubt it was because of the tires.