What does it mean when a STEM researcher has a lot of "solo" publications? This is related to advisor selection for North American universities, STEM majors.

I noticed some profs tend to have a lot of "solo" publications, meaning that he or she is the sole author of a journal/paper for over the span of quite a few years. This tends to be from earlier on during his or her career as a researcher.

Is this common in academia? What would cause this to happen (not saying that it is a bad thing) and does it hint at anything about the particular researcher?
The normal number of co-authors, if any, is very specific to the different (sub-(sub-))disciplines. In my specific sub-discipline it is normal to have a number of solo publications and a number of publications with one, maybe two, co-authors. But in the same department we have people who do primarily qualitative research and they tell me that it is very unusual to have co-authors and they cannot imagine how that would work. From the outside we would both be sociologist, but even within disciplines there are big differences. In other disciplines it is normal to have 5 or more co-authors. So the number of co-authors per se does not say much without also considering the sub-sub-sub-sub-discipline that that person works in.