Those are guys that quit whatever team they're on at season's end due to blown promises and/or lack of playing time. Sometimes those guys can be decent for the level they show up at (if their former owners didn't wreck their work ethics...), but, yeah, they tend to contact everyone within a set radius of them that meets their criteria (I think they also have to match either the Off or Def, but that might just be a coincidence for the ones I've bothered to peek at...).