Mickelson has always been impulsive, both on the course and off. It seems to be "who he is." Unlike other athletes who carefully manage their images, with Mickelson it's always been "what you see is what you get."
He's gotten away with shots that more "prudent" players wouldn't dream of making because he's so darn good and has won more than 50 times in the US and Europe. But, with all his talent, it took him 12 years on Tour to win a major. Then he seemed to settle down a bit as he got older and won five in eight years. But, the US Open has always eluded him (T2 * 6).
He's also a very serious gambler...you don't think his foursome with Brady, Fowler and their Investment Banker friend had anything less than six figures on the table do you (with Phil and Ricky playing to"plus-something" and the other two to "minus-something")?
In addition, he skated on the slimmest of technicalities in a serious Insider Trading case.
So, what he did on Saturday is pretty consistent with who he is. He knew that another US Open was out of his grasp. He was frustrated and acted impulsively...but in a calculated way. He knew that the two-stroke penalty under the circumstance would a DQ and already knew the answer when he called the head of the USGA to ask whether he was disqualified.
The ball he stopped was going to roll way off the green and give him a problem coming back, but not a problem that a golfer of his caliber couldn't handle with two additional strokes anyway. Then, he lost his cool and did what many of us have done playing with our buddies without money on the table.
However, if the ball was somehow in danger of rolling OB or into a Water Hazard, I think that the DQ rule would have taken precedence.
As his wife said later, it wasn't his "finest moment." But, I still love Phil.