...I have never had more than 400 open ES and NQ positions combined, very short-term day trades, in either account at any time and I have always had more than enough free cash in those accounts to cover this margin.)...
This could be the crux of the problem. Have you been following the margin increases from the beginning of the year?
"...I also had both buy and sell flattening orders that were not at the exchange..."
So as far as AMP knows, you had open orders with no stops, because if they ain't at the exchange, AMP would not know about them.
Either way, their risk algos saw that you had a maximum of 400 contracts of ES/NQ combined? Well...
NQ initial margin is $16,500 per contract.
ES initial margin is $13,200 per contract.
So let's say you had 200 ES@ 13.2k. $2.64 million cash required.
And 200 NQ @ 16.5K? $3.3 million cash required.
That's 5.94 million bux you'd need to maintain those positions. And that is if your positions are at BE! If they move many many points away from your entries, well, you do the math.
Their discount margins are meant for daytraders. With any firm you need to trade as if you are under initial margins, "just in case", not their uber day discounts, when trading any real size. I think that is what their risk management system did here, and alerted their risk department. Without a trade history/NLV view during some of your typical days, we'll never really know.