Ever felt KSP needs more explosions? Me neither really, but space debris is a problem in real life, and now it can be a problem in your game too!
When/if you start leaving debris in orbit, your craft may encounter a "micrometeroid", which will explode a random part of your ship, unless the game considers it to be shielded from the airstream.
Debris only counts (and therefore explosions can happen) if:
1) It's Periapsis is above the atmosphere height (or 5000m for airless bodies)