Balancing on two wheels (or two feet, for that matter) is a dynamic situation. There are hundreds of little muscle movements, balance shifts, bike moving under you, variations in road surface, etc. If there is a disruption at the wrong instant, you can go down quickly. So, for example, if your wife's balance was shifting to the left (for whatever reason - dip in the road surface, easing a saddle pressure point, whatever, something we all do thousands of times during a ride), what should happen is she should turn the handlebar to the left to keep the bike under herself - she would do this automatically without even noticing it. However, if her tire hit a crack at the wrong instant, making her unable to turn the handlebar to keep the bike under he, she would fall to the left. There's no way of knowing if that was it, but it's a possibility. Basically, in this situation, she would have just gotten unlucky - she was at the tail end of the distribution, all of the random errors just stacked up the wrong way to take her down.
(edit: in skiing, we call this a "snow snake", one instant you're skiing along, the next instant you're down)