That was the last one. A French spy satellite lost contact, and they asked the Brits about it. The British equivalent of NORAD looked back over its records and found an occurrence of intersecting orbits.
It was determined that a dead satellite (also French, iirc) clipped the bottom of the spy bird, shearing part off, and sending it tumbling.
When you think about space being big, you have to remember that there are desirable orbits, and they do get crowded. Also, gravity sucks, and those objects get pulled down into much narrower "corridors", increasing the chances of collision.
Since global warming isn't happening, the atmosphere isn't expanding often to create drag on all the defunct objects and junk, to help to clean up some of the stuff.
They recorded the whole aftermath in Gravity.