Yeah OP called it a pension when it was really a 401(k) — but Enron had some sort of setup where employees could choose to buy Enron shares at a discount into their 401(k) but importantly, Enron’s “employer contributions” were also in the form of Enron stock, which couldn’t be sold by the employee until they turned 50. Then when the company started failing, they instituted a trading blackout so employees couldn’t sell their stock for weeks before the bankruptcy.
Of course, there was a corporate pension as well that failed and was was also largely in Enron stock. So now those liabilities are owned by the PBGC.