liquidity and Yellow buybacks
When debt comes due, most companies can "roll it over" (issue new debt to replace it). That's not an option for Yellow any time soon. Also, their $1 billion bank credit limit ($250 million loan + $750 million revolving facility) will presumably be reduced when it expires in 2013. When debt comes due, Yellow needs to pay it in cash.
If liquidity is a big problem, Yellow should focus on retiring debt that comes due soon (pref A, bonds with short maturity). If it's no problem at all, they should buy back securities that have the highest effective yields (pref B, C, D, 2036 bonds, common). Their actual buybacks are balanced between these approaches, saying that in their judgement liquidity matters but it's not a big worry.
The market is pricing Yellow debt and equity as if the company is in imminent danger of collapse. Yellow's finance department are not acting like that. They have a huge pile of cash from the Trader sale and large but shrinking positive cash flow.