Most members of Congress engage in Insider Trading, all the time.
It is one of the primary vehicles by which many members enter office with little net worth, and on a salary of $174,000, leave office a few years later worth tens of millions of dollars.
Was this case just too blatant for the Dept of Injustice to ingore? Was this a message being sent to members of Congress that the noose is tightening via President Trump?
The former Speaker of the House, John "Wiskey" Boehner told the press that Insider Trading laws "didn't apply" to members of Congress.
https://theintercept.com/2015/05/07/congress-argues-cant-investigated-insider-trading/
Was the indictment today because he tipped off his son and lied to investigators? Or is Sessions sending a message to Congress that we can sink you all?