There is never a good reason to endanger others. But I'd think most cyclists should be less than surprised that many non-cyclist have no love for cyclists. In fact many people actually hate sharing trails, paths, and roads with cyclists.
I don't think we cyclists deserve the negative rap we get from so many. But with decades of finger-flipping, and rude behavior, I am not at all surprised that some crazy old lady threw some sticks and rock on a trail.