I believe Hermione had not thought of Umbridge's non-human bigotry when she led her into the forest. She was thinking only of the previous exchange she had witnessed between Hagrid and the centaurs earlier in Order of the Phoenix (chapter thirty, "Grawp"):
"As are you, human," said Bane, "coming back to our forest when we warned you --"
"Now listen ter me," said Hagrid angrily. "I'll have less of the 'our' forest, if it's all the same ter you. It's not up to you who comes and goes in here --"
"No more is it up to you, Hagrid," said Magorian smoothly. "I shall let you pass today because you are accompanied by your young --"
"They're not his!" interrupted Bane contemptuously. "Students, Magorian, from up at the school! They probably have already profited from the traitor Firenze's teachings..."
"Nevertheless," said Magorian calmly, "the slaughter of foals is a terrible crime...We do not touch the innocent. Today, Hagrid, you pass. Henceforth, stay away from this place."
Furthermore, from later comments, they took this exchange to mean the centaurs wanted no humans in their forest:
"Hagrid," said Hermione breathlessly, skirting the patch of nettles they had passed on their way there, "if the centaurs don't want humans in the forest, it doesn't really look as though Harry and I will be able --"
Therefore, I have always believed the explanation of the scene of Umbridge confronting the centaurs to be that Hermione had not given complete thought to what would happen when Umbridge met the centaurs. Her goal was only that the centaurs would try to chase Umbridge off for being an adult human in the forest, while they had already said they won't harm children. She was then shocked, especially due to her identity as a non-human rights activist, when Umbridge started insulting them to their faces, and wanted to defend them.
And if you think about it, Hermione would have been not a very good non-human ally, indeed, if her first thought had been to bring a racist to the centaurs' forest so that she would hurl epithets at them. Although, as the centaurs pointed out, Hermione was still guilty of the lesser offense of intending to use them, however harmlessly, to achieve her own ends.