“Why was I sued by the car insurence of a parked car that I hit only after another car crossed over head on into my lane of traffic?”

The short and  answer is that you were operating the car the struck a parked car.  But you have have an excellent point and your defense of this case illustrates the Emergency Doctrine.  Under the Emergency Doctrine, a driver is not responsible for his actions if they arise out of an emergency situation not of his own creation.  It is not resonable that a car would cross over into oncoming traffic.  This emergency was not your fault. Therefore, any damages caused by your striking a parked car is attribuatble to the person who crossed over into oncoming traffice and created the emergency situation. Your car insurance will handle the defense of this case and you should notify the insurance company promptly. 

But the application of the Emergency Doctrine rule can be tricky.  Suppose you hit that same parked car because it was snowing and the roads were treacherous?  You did not create the snow.  However, in that case, the weather is easily discernable by anyone driving.  Hence, the driver created the emergency situation by driving in bad weather and at such a speed that he could not maintain control of his vehicle. Therefore, it is up to the person driving under wintry conditions to exercise extreme caution.