ACA's where Big Shitpile would pretend to shift a big bunch of risk so they could grow their shitpiles ever higher while pretending everything was fine. If ACA goes under, all that risk goes on their books and the shitpile does come a tumbling down.
Essentially financial institutions had been dumping all their risk on ACA which was providing "insurance."
While there's no new news out about ACA, their stock plunged 24% to 54 cents today, giving them a market cap of 18.9 million. ACA insures about "$70 billion" of Big Shitpile and other assorted piles.