Speedbird, a blown seal or nitrogen loss results in the oleo being "flat", when the nose gear collapses, it's the entire strut giving way leading to the nose of the a/c contacting the ground causing the prop to get bashed.
It's usually caused by people approaching at too high a speed and forcing the a/c down onto the nose wheel instead of going around or holding it in the flare. A very common and expensive occurance!