I don't have any special knowledge of this situation... but because I started my career in public accounting at Arthur Andersen, I always followed the story of their meltdown.
Here's one thing I understand based on comments from former AA personnel: The partners historically had accumulated a large partnership capital account balance that, as part of their retirements, got annuitized when they retired. A senior partner, e.g., might have $6M in his or her capital account.
It was "seemingly" risky to have so much wealth tied up in one asset, but hey, it was Arthur Andersen, widely-regarded as the best of the Big 8. What could go wrong?
P.S. The actual pension plan for rank and file employees should have fared much better.