Remember last April, I was talking about space junk and the possibility of a "runaway Kessler Syndrome"? This is when the amount of space junk flying around means anything in orbit will be destroyed by debris.
In the worst case scenario, humans could be stranded on Earth because of the Orbiting Cloud of Death. (I ought to write a SciFi novel with that premise.)
If there's going to be a Kessler syndrome, the most likely cause will be debris created by satellites being destroyed by existing space junk. Wikipedia lists at least five instances where satellites have been damaged or destroyed in collisions with space junk or "micrometeoroids". As well, space shuttles and the ISS have been hit and damaged by tiny bits of debris.
This photo was taken from the Space Shuttle in 1998. I think it's a thermal blanket from the same mission, not an alien spacecraft. Seriously. Search for "Black Knight satellite conspiracy theory".
Another source of space junk is satellite "shootdowns". Russia, China, India and the US have all demonstrated that they can target and destroy satellites using "hit-to-kill", or kinetic strikes. These tests have caused a lot of debris.
Interestingly, a bit of debris from the Chinese shootdown test was (allegedly?) responsible for damage to a Russian BLITS satellite in 2013.
Collisions between intact satellites are rare - in fact, as far as I know, it's only happened once, on 11 February 2009. A long-dead Russian satellite collided with an operational Iridium GPS satellite. The closing speed was over 35,000 kmh, and the debris was immense.