The root cause is people are bad at projecting the future. Defined benefit pensions are projecting economic conditions decades into the future, and they’re simply not true (at least not until the government juiced the numbers to offset the anemic growth due to outsourcing and automation). But then that’s just inflating away one’s debts, and the recipient is still screwed.

The second problem with defined benefit pensions is agency risk. Anytime you have a big pot of money, it invites corruption, and in the case of DB pensions it’s very easy for people in power today to tilt the scales towards themselves at the expense of those decades behind them. And so they do, by voting themselves lavish benefits, voting for politicians to enhance benefits, voting to hire the actuary that understated the costs, etc (that stuff is more so in taxpayer funded DB pension land).

For multi employer defined benefit pensions, the simple matter is US economic growth in their businesses didn’t pan out. A lot of businesses contributing to the funds consolidated, got automated away, outsourced to cheaper countries, etc, and so a lot of the assumptions in there don’t hold true and so they come up short.

But keep in mind, every time the government bails someone out, you pay little by little by your USD being worth less and less.

It depends on which type of pension fund and is generally more complicated than a single reason.

One big general cause of failure was that companies were allowed to stop paying into the funds without paying enough into the fund to cover their employees who would receive.

However, the particulars don't actually matter here. No matter the reason, it will have been the responsibility of the employer, fund manager or government regulators. The average Joe, who will lose his retirement if the pension is allowed to go bankrupt, didn't have enough control of the fund to be held responsible. As such, punishing him will do nothing to de-incentivize this from happening again.