F4 Tricking Baserunner

[aschramm]

I'm watching this video, and I vaguely hear "Dude, It's foul". Actually after watching a few more times, I think that was yelled on an adjacent diamond with another game going, as you can hear another hit with a metal bat at 0:12 and 0:15. But even with that, I don't understand why the runner just gets up and heads back to first, when I believe he was able to see the ball go past him behind 2nd base.