Student loans are supposed to be a bulletproof asset class for securitization. The debt cannot be discharged in bankruptcy; moreover, the government guarantees most loans. Investors can be sure they will be repaid.
But as recent events have shown, they can't be sure exactly when they'll be repaid. And that uncertainty has roiled the normally staid market for bonds backed by Federal Family Education Loans, prompting downgrade threats from the rating agencies and a steep sell-off over the summer.