If you have some stupid custom software that's making bad private keys you would never notice until you go to use them (or test them). Even more plausibly, they mention in 2011 to be using a custom system like Shamir's Secret Sharing, maybe they suddenly realized that their shared secrets are all bad?

Not saying that's what happened, but it's not an unreasonable thought. It's more reasonable than losing 750,000BTC to an issue that could have been picked up by even the most basic accounting.