"Unless the woman is nuts, I am pretty sure that the attack was not unprovoked. Perhaps she responded in a public situation because it was the only place that she felt safe enough to attack him, because she knew that he wouldn't hit her in public."
REPLY:
Could be, but it's all a guess at this point.
I hope that she gets medical help and a good place to start would be her family doctor.
The article on smoking Gun has much more info. It says that she showed the arresting officer a large bruise on her thigh and that the husband admitted that she was just angry from an earlier incident and that she really didn't harm him.