bike forks fold backward from frontal impact or picking up something in the wheel that jams against the back of the fork. I thrashed a suspension fork once by catching a big stick in the spokes. The road forum has a recent "squirrel in the forks" thread that has interesting pictures of, um, squirrels in forks and the resultant damage. It clearly wasn't an alien death ray, and whatever caused the wheel to cease rotating is apparently gone missing. BTW, the fork will probably fail in accordance with any stress risers that were built into it at creation: if the cinelli crown introduces stress risers, then the failure will reflect that, but the fork crown most likely did not cause the failure...there are about a gazillion bikes using that crown every day, and I haven't ever seen or heard of a crown-related failure before (that doesn't mean that the crown did not cause the failure, only that I would look for more everyday causes before seeking a rare solution to the puzzle).

I honestly have never seen a fork fail from fatigue or weld failure in action. I have a small collection of forks that had at some time in the past been crashed and probably should have failed, but didn't, and I replaced 'em with something better. A Bianchi with a crack in the steel traveling upward from the dropout three or four inches, a couple of treks with cracked or badly swollen steerer tubes, a suspension fork that had the shock legs wear the fork crown oblong. None of 'em failed in action.

Have you disassembled the headset/fork to see if there is damage to the steerer tube?