Bugger! Glad you're alright. I had this happen to me last year, fortunately at low speed and I managed not to fall off. FWIW, I think in my case it was partly if not entirely caused by me knackering the bars when installing them - they were tight in the stem, and I kind of back-and-forth twisted them to get them in. The scratches seemed superficial, but with hindsight, I think they might have created a weak point from which a crack gradually developed. I could be completely wrong, but I certainly didn't take the chance when installing the replacement bars. I made damn sure the stem gap was wide enough.