It's fraud - people refusing to tell or put things down on official financial reports, writing false statements, signing false financial reports, using non GAAP, unusual accounting methods, etc. Also, audits and due diligence didn't uncover the magnitude of the problem. It took a whistleblower to get the story out to upper management.

https://www.thestar.com/business/2017/02/14/toshiba-chairman...

this article outlines the contract issues, but there were other issues at play at the same time

http://www.reuters.com/article/us-toshiba-accounting-working...

https://www.recode.net/2015/5/5/11562326/hps-5-billion-fraud...