What would cause the front bump stops to disintegrate like this?

[Racer Boy]

The rubber used in the suspension parts on these cars is junk, which seems to be common with German cars. Rubber degrades over time, and whatever specification of rubber that Porsche used on the front bumpstops degrades even faster than the rear rubber. As TeamOxford mentioned, your car is 20 years old now, so they are falling apart.