Discover closed my account with the message "we are no longer able to meet your servicing needs." No clue about why - I haven't had an add since late last year. Pretty normal usage on it.
I continue to suspect that companies do periodic audits where they query their databases for what they consider to be risky behavior. They then either human review or (less likely) do mass closings.
If this is accurate, then there could be a long while between when one does what they consider risky and the account closure. It also means that "normal usage" may not inoculate one against account closure.
I continue to think that the total number of AU's added to a line, perhaps limited to those that don't reside at the same address or share the same last name, would be the search criteria that they use.
I also don't think they expend much energy or effort on this kind of thing overall, since the direct cost to them is pretty low. Their main business risk would be loss due to fraud or punitive damages by not complying with federal restrictions on foreign nationals subject to sanction or something like that.
@Shade00, there also is the possibility that the reason for closure may not be related to AUs. It's possible that some other behavior is what triggered the closure. I don't know about you, but I typically do multiple things on each credit line that the CC companies don't like.