Because “the enemy of my enemy is my friend”. The Faithless just took one heck of a beating from the Horde (lore wise) and now the Vulpera join the Horde. Id be looking to get some payback if I were them.
Look at the Forest Trolls in TBC after the Blood Elves joined the Horde. The forest trolls were once Allies to the Horde and the moment the Blood Elves joined they immediately became heated enemies.
Who you form an Alliance with can impact your enemies greatly.