1. He should know which outfielders to run on.
2. He was running with his head down as he was rounding 3rd, should have been watching the coach, so intentional or unintentional he F'd up not looking or blowing through it.
3. And why risk it, would have been 1st & 3rd nobody out, and 2 batters, one being yourself, just rocked Scherzer. So you are probably scoring from 3rd anyway.
So Walker F'd up one way or the other....or the other.
Watch the 3 min mark, and especially on the reply.
https://www.youtube.com/watch?v=ojfJo8x6y_Q
He's not even looking at the coach. Coach had it well up before he was running past him.