Why do they [European financial institutions] hold so much Greek government debt? Because under Basel II, implemented (outside the United States) in 2007, Greek government bonds, rated A-, had the same 20 percent risk weight as AA/AAA asset-backed securities in the United States. That is, until S&P downgraded Greek debt from A- to BBB+. That raised the risk weight to 50 percent, suddenly requiring 60 percent more capital from banks holding Greek bonds.
This appears to be the reason that the possibility of Greek default has led to fears of another banking crisis.
via Causes of the Crisis: February 2010.
The 20 percent risk weight required banks to only hold $2 of bank capital against a $100 security — at the 8 percent Basel rate for adequately capitalized banks — allowing 50 to 1 leverage compared to 12.5 to 1 on normal bank loans.