My leading guesses would be somebody’s mistake leaned in the same direction as his fraud and caused an audit. Or the cumulative skim made one of the bank accounts noticeably short. Keeping your skim under materiality can help you avoid the auditors for 1 year. But if you keep doing it the cumulative effects eventually wind up material. And of course the longer you get away with something the bigger the ego gets. Cause the smart person who successfully skims $5 mill retires.
Greed got him...and like a dummy he flaunted it with the wild spending.