Originally Posted by
Lac
By the time the burning of Teldrassil happens, Baine is already on very thin ice with Sylvanas. Unlike his father, Baine doesn't have strong ties with any of the Horde leadership and is more isolationist than the Tauren have been in the past. The Tauren have always been more moderate than the rest of the horde, preferring peace and harmony over conflict but they have waged war with the Horde out of loyalty and because they believed the horde to be honourable. When the horde lost its honor under Garrosh, his father stood up against the warchief and paid for it with his life. Before Darkshore, Baine stood up to Sylvanas after the massacre at Arathi highlands and she threatened his life and the lives of his people. He knows that she will follow through with her threats as she is without honour.
The Derek Proudmore situation is the straw that breaks the camel's back. He was pissed at what happened at Darkshore (burning) and Undercity (blight) but had to protect himself and his people. He had no control over those events as he didn't have prior knowledge but with Derek, he knows what Sylvanas is planning and that it is immoral. He can do something about it and standing by would be wrong, in his eyes.
Personally, I like that the horde are having to question what it means to be 'The Horde' and that characters like Baine and Saurfang have concluded that in order to protect what they believe 'The Horde' means, they have to betray the Warchief. It makes the faction more interesting and less black and white, clear cut.