Select to add only the specified models and drawings. Otherwise Add will process all the parts, and if any of the parts have been Added to drawings it will bring in the drawings, and with the drawings all the parts that are shown on the drawing.
The relationships are usually correct, but sometimes it is just sloppy work - such as Save-As on a drawing to create a new drawing for a different part; the user fails to delete the old model and now the new drawing is linked to an otherwise unrelated model. Usually I've seen this when family drawings are made - to save a little time rather than copying and pasting notes and symbols. Other times it is from users faking views - they add a piece of hardware for some detail rather than creating a detail view.
I often use Advanced Add because it shows all the items that will be brought in, unlike Add, which tends be make for a complete surprise.
As to finding the culprit - look at the drawings that were brought in using Commonspace and see which have models that are directly referenced, rather than Default. To fix part of it, if it is possible, you need to open the drawing(s) and delete all the drawing models that aren't actually used. This will prevent Add from linking in the drawings based on the unused models and then the rest of the parts related to the drawing.
I hope this makes sense.
