The problem that triggered his being investigated was a series of suspicious money transfers. They had no way of knowing it was to hide his prostitution bills, but the pattern triggered IRS alarms in software that tracks bank transactions of everyone--but most especially those of "politically exposed persons" such as Spitzer.
Software that was put into place at the direction of: Spitzer.
How could he--of all people--not know his bank activities would be noticed and would result in his being investigated?
SAM: What's new, Normie? NORM: Terrorists, Sam. They've taken over my stomach and they're demanding beer.