I had the steerer tube fail on our tandem's fork almost 40 years ago and still have a slight scar on my hand as a reminder. Looks like one of the balls in the lower race may have cracked and the pieces started to wear a groove in the steerer tube until it suddenly gave way entirely and the fork/front wheel separated from the bike. The bottom of the head tube impacted the pavement and my chin and hands took most of the impact. The back of the bike stayed up and my wife wasn't injured. A frame builder replaced the steerer tube and reshaped the dented head tube and we still have the bike.