I often share the reason in PM with the user in question, rather than in public.
Most common reasons for being removed:
Legitimate Negative Trust.
Extreme Post Bursting.
I always write something in the "Comments" section of each user that is affected by something, for example if someone is just writing one liners I give them a warning by typing "Stop posting only one liners" and if this continues for several weeks without improvement I usually remove the user. There are other things too, like minor post bursting etc.
Just make sure to always check the spreadsheet.