David Sokol, the chief of MidAmerican Energy who saved Constellation Energy from bankruptcy in the thick of the 2008 financial crisis, has abruptly resigned from Berkshire Hathaway, MidAmerican's owner. Sokol was the guy who negotiated with Constellation boss Mayo Shattuck and extracted a huge ransom from Constellation shareholders for the rescue.

Andrew Ross Sorkin reports some stock transactions by Sokol that might be perceived as front-running Berkshire shareholders. Berkshire recently announced it would buy Lubrizol, and Sokol had been buying Lubrizol stock earlier. Sorkin:

Mr. Sokol purchased 2,300 shares of Lubrizol on Dec. 14, which he then sold on Dec. 21, according to Mr. Buffett. On Jan, 5, 6 and 7, Mr. Sokol bought 96,060 shares “pursuant to a 100,000-share order he had placed with a $104 per share limit price,” Mr. Buffett’s statement said.

Here is an excerpt from a good Fortune profile of Sokol after his staredown with Shattuck:

Sokol phoned the office of Constellation CEO Mayo Shattuck III, who was in an emergency board meeting. When his assistant answered, Sokol told her he'd like to speak to him. The secretary replied that if she interrupted the meeting, she might lose her job. Sokol replied, "If you don't interrupt the meeting, you might lose your job."

Sokol boarded a Falcon 50EX and sped to Baltimore. He met with Shattuck and struck a deal that evening to buy the company for $4.7 billion, staving off bankruptcy.