There is a good article on
Reddit as to why Sylvanas does what we see in the War of Thorns.
As for the Tauren, what have the Night Elves even done for them? Did they help them when the Centaurs threatened to wipe them out? Or, maybe, when the Quillboar were attacking due to drought after the Cataclysm? No? Only united forces against world-threatening enemies? Thought so. Similarities in faith only go so far and last so long. Look at Elves and Humans - their faith is also similar and it didn't stop them from spatting at one another even before the Horde has been formed.
My Tauren (druid, by the way) personally has been attacked by the Alliance during their ambush and assassination attempt in Stormheim, right in the middle of her quest to get the Aegis of Aggramar to save the world. The Forsaken forces sent to help her in the quest had to be redirected to be fighting Gilneans instead.
She has also been in Silithus and seen dead Goblin Miners and SI7 spies (the same SI7, by the way, that got infiltrated by the demons and lead united armies of the factions into the trap on the Broken Shore, causing massive losses and deaths of Varian Wrynn and Warchief Vol'jinn).
Now the Warchief says there are SI7 spies sneaking around Orgrimmar - and, as the Night Elven fleet moves out to Silithus according to the false information Sylvanas planted, I have no reason to not trust her that a war is coming and decisive action can stop it before it brews fully.
Yes, the loss of Malfurion Stormrage would have been a great blow to the whole world, and I think my character would not have been able to kill him in Saurfang's shoes, but war is war. And Malfurion's not on the same side this time.
Edit: There is also a
big thread with various Horde druids explaining their PoV on the official forum!