pensions aren't killing them.
they had pension obligations
which they then underfunded, leveraged, borrowed against, and played various accounting games with.
at that point "Hey, it's broken!" and they either get their pension bailed out by PBGC or get a judge to change their contractual obligations.
corporate greed just found a weak spot in their pensions for them to slush funds around and overextend themselves.
the huge stock market bubble helped a lot of shady accounting: "look, with the market up 200%, we only need to fund 25% of the pension!" was a legal policy. After the crash... they'd been invested 25% of what they needed, into the riskiest, most devalued securities.
(I worked briefly for PBGC.)