Why are auctions failing on auction rate securities?

Accrued interest explains. It's interesting.

Update. Bloomberg has an article here on recent auction market failures. Another factor they mention is the unwillingness of banks to support the market by underwriting the bonds. Given some of the yields available and the good credit quality of the (mostly muni) issuers, I'm surprised the hedge funds have not stepped up to the plate. Anyone fancy a revolving CDO of auction market securities structured like a credit card deal with early am and such like?