These dratted things are almost always temporary files (e.g. browser caches, temp Photoshop files) that are loaded in memory at the time the machine crashes. Normally, they will be cleared on reboot, but if the machine has crashed, the proper flag is not in place at restart, and so the poor OS tries to help you out by "rescuing" these files.
You, however, indicate that your machines have not crashed when this occurs. Try to findout which app has left these droppings behind (try a "Get Info") and be sure it is operating correctly. If this doesn't help, you could always write an AppleScript to empty the Trash on startup. That should keep the little devils from clogging the works.