Here's the thing about leverage:
Go back to our hypothetical bank.
Assets = 100 dollars
Liabilities = 90 dollars
Surplus = 10 dollars
Now, you want to grow your bank, so you decide to borrow money, say 50 dollars, and use that 50 bucks to buy new assets. For a bank those assets are loans, or in this case CDOs or MBSs.
Assets = 150 dollars
Liabilities = 140 dollars
Surplus = 10 dollars.
Your ending surplus is exactly the same, but you are now heavily leveraged. If the value of those assets remains stable, you have probably increased your net income by around 40 to 50% as well, making your shareholders very happy, and earning yourself a nice fat bonus.
What happens when the value of your assets declines by that same 11%?
Assets = 133.5
Liabilities = 140
Surplus = 133.5 - 140 = -6.5 dollars
Your bottom line measure of insolvency has gone from - 1 dollar, to -6.5 dollars.
In fact, if one does the math it only takes a 6.67% decline in asset value to make you insolvent. (10/150)