"It just looked real funky ... and with the way our players reacted and seeing the runner on the grass, it just look awkward," Black said. "He [Diaz] had his point of view."
With Brandon Crawford on first base, Tanaka at second base and Andres Torres at the plate, Padres relief pitcher Brad Brach threw a ball that bounced off catcher Nick Hundley's glove as both runners attempted to advance.
Hundley threw to first baseman Yonder Alonso, who then threw to Headley. Headley ran Tanaka back to second base and first made contact on the grass near the edge of the dirt after throwing the ball to second baseman Jedd Gyorko.
"Obstruction is when you're not in possession of the ball and not in the act of fielding the ball, you hinder the progress of the runner. Neither of those two apply to him [Headley]. He has to disappear at that moment. It was a textbook obstruction case," crew chief Mike Winters told a pool reporter after the game.
Diaz immediately signaled for obstruction as the Padres vehemently protested. Black argued briefly with Diaz and was ejected, his second of the season.
"We looked up the rule, but from our vantage point, he was three or four feet in on the grass," Gyorko said. "It was just one of those weird plays."
According to baseball's Rule 7.08, a runner's baseline is established when the tag attempt occurs and is a straight line from the runner to the base he is attempting to reach safely. Diaz felt Tanaka didn't veer from his path.
"When Headley threw the ball to second, before Tanaka could turn around and go back to third, he ran right into him," Diaz said.
The Giants scored five runs in that seventh inning and eventually won, 10-1.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. Jamal Collier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.