The first cause is the meltdown in the subprime market and what it did to the value of securitized derivatives consisting of bundles of mortgages and other hyper inflated assets. There has been a near total collapse of these securitization markets since subprime mortgage defaults began to surge. Unfortunately these securitization markets are what most major financial institutions had and have large positions in. Some of these positions were on-balance sheet but also quite often not. Thus you are seeing large on-balance sheet losses and the market panic caused by the spectre of even more massive off-balance sheet liabilities at many, if not all, major financial institutions.
No one really knows what these toxic derivatives are worth. Thus this first cause turns on finding a way to value (price) these derivatives that a gun shy market can trust. No faith in value equals no sale, no matter how many Nobel Prize winning equations you have on your side. In the meantime in the face of these paralyzed markets the government simply needs to seize the institutions that are illiquid due to their huge unrealized losses on their derivative holdings.
The second amplifying part of the econapocalypse is the inability of the sellers of insurance (read AIG) upon debt and derivatives to meet what are essentially margin calls on their positions. As the value of the debt and/or derivatives AIG was and is insuring plunges, AIG is contractually obligated to post cash collateral to its counterparties based on factors such as the risk of default and loss in value.
Worse, AIG CDS contracts are the parchment barrier between liquidity and illiquidity for many financial institutions. If AIG cannot make its contractual payments on these policies many of these institutions will no longer be able to keep their derivative losses off there books, leading to illiquidity and defaults (that AIG insured) and margin calls because of the change in credit risk. The formerly lucrative CDS echo chamber thus grows into a roar that no one has been able to quell. The administration's action on this front has been weak and is cause for concern.