The forest trolls left the Horde like right after Orgrimm abandoned the siege on Quel'thalas in WC2 tho. Or was that just a retcon later when the lore was expanded upon?
Thrall invited the forest trolls to be allies, but they refused because the Horde failed them in the Second War. Only the Revantusk Tribe joined the Horde again, and they saw the rest of the tribes as traitors.