All of the replies make sense, and are good reasons for Amex to cancel you.

But one combination of factors seems especially suspicious: You've got multiple AU's (at least two - were there more, currently or in the past?) AND "they" are the ones "causing trouble".

This suggests several concerns, mostly involving, "Who ARE these AU's?"
You don't seem to be in frequent contact, in addition to their being "problems" for Amex.

If they are paying their own bills, why don't they have their own accounts, perhaps guaranteed by you, IF you really trust them... and if not, why are they AU's?

Anytime we've had AU's, the bill has been paid with the primary, including the AU charges (but perhaps this isn't the norm??).
But our AU's have always been people like a spouse or adult (or almost adult) children. That is, they were people we knew very well, and we were also aware of their use of the card. (Even for children away at school, we could track their usage; no surprises...to us, and certainly not to Amex or other card vendor.)
For occasional others, there were occasional *separate* accounts (including joint), but they didn't have access to "MY* account.

If the others were indeed appropriate business users, why not give them separate accounts (whether labelled as "corporate cards" or not)?
Then, for example, the disputes, etc., would not have directly affected *your* account.

All of the above might make the "AU's" seem like some sort of sketchy arrangement...and that is probably a huge red flag of its own.

But mostly, it was YOUR account, and you let it get out of control, more than once, and in more than one way.

GC