Variance in academic fecundity during and after graduate school
I'm sure there are more reasons, but here are two positive reasons.

Some people get a job that stresses teaching more than research and so they put their focus there. It can be a rewarding life, actually. Lots of teaching and a bit of research. Maybe less stress. Of course this is an unlikely scenario at an R1 university. 

Some people, after gaining tenure and leading a fairly large group of graduate students in a lab or research group, are willing to let the students gain the glory and don't add themselves as co-author, even though it might be appropriate. This isn't a good path before tenure, but can be afterwards. So they are doing research but, by feeding ideas to them, the students gain the credit for it.