I'm guessing that everyone, or almost everyone, knows that California Chrome did not win the Belmont, nor did that beautiful horse win the triple crown. One of the owners was not happy with the outcome and wants a change in the rules. That owner wants only the horses that qualify for the Kentucky Derby to be eligible to compete for the triple crown race. Many of the columnists and commentators are calling this sour grapes. It's not.
Remember Aesop's fable. The fox saw a bunch of grapes, too high for him to reach, and after trying and trying to snatch them, the fox decided that "those grapes are probably sour anyway," and the fox stopped trying. Sour grapes, when used in today's language, means that the very prize that you were seeking is not worth the effort.
This is NOT what California Chrome's owner had in mind. He was angry that he lost. He was angry that his horse did not win the triple crown. He is now trying to change the rules so that, if those rules had been in place, his horse might have won. He is the poster boy for POOR LOSER.