Upon examination of the headset, the mechanic noticed a cracked steerer tube just above the crown. The suggestion was first a warning that the bike should not be ridden and that a replacement should be installed. The fork blades itself and the dropouts and the crown are ok. So is the head tube and bearings.
First, what is the likely cause of this type of fracture. Its maybe less than an inch above the crown and is less than half the circumfrence of the steerer tube.
The bike is near 2 years old with about 7,500 miles of riding. This summer going downhill for about 2 miles, the bike started to shimmy, wobble uncontrollably at speeds of maybe 35-40 mph. By braking I got it to slow down but still managed to eject head first over the bars as I came to a near stop speed. Initially, that wobble felt more like a rear flat but the frame shook uncontrollably in short time.
Second, does this type of fracture occur frequently? Is it due to riding or what?