It does seem to be random, but as with all random events, luck seems to play into it too. It seems as though the same person gets hit with that email time and time again. I have gotten into the habit of checking my people's progress before opening the email. If they are just barely started (or a Master) I will go ahead and open it. If it takes away progess, it won't take much that way, but if it happens to be the automatic promotion email, they get more benefit from it.