Here's another link to that event, that also discusses a collision between two satellites:

http://www.space.com/8334-junk-space.html

The claim is that the China missile test and this collision are responsible for 60% of the number of objects in orbit.

Even though space is vast, the number of useful orbits is limited. For example, the A-train (http://en.wikipedia.org/wiki/A-train_(satellite_constellatio... ) is a parade of remote sensing satellites in sun-synchronous orbits (as they look down, it is always within a few minutes of noon). So this makes inadvertent collisions more likely.

Slightly related, here's a close asteroid passby that happened last summer:

http://neo.jpl.nasa.gov/news/news172.html