When redstone powering a command block (from diagonally above) has its power level changed (i.e from source being 10 blocks away to 5 blocks away) the command block re-executes it's command.
This doesn't always happen, and I'm not sure what conditions cause it, but I have been able to replicate it a couple of times by accident. This bug will occur for a number of times and then disappear. Closing and reopening minecraft doesn't fix it. I can open another world where this doesn't happen and return to the first and the bug will still be present. Destroying and replacing the command block does seem to fix it.

In the screenshots, the bug appears in the last two pics. When I place the torch in the third pic, the command block executes, and when I remove it in the forth, it executes again.