To the trolls, the spirits of the dead are just as much an entity as living creatures -- greedy and dangerous entities that are jealous of those still alive in corporeal form. These spirits miss the land of the living and require sacrifices to appease them. This is why trolls sacrifice their enemies -- to keep the spirits of the dead satisfied and happy, so that they don't wreak havoc. As for cannibalism, the trolls believe that by eating the flesh of their enemies, they are also consuming their spirits -- or at least damaging the spirit enough that it will be rendered unable to commit any acts of vengeance.
This makes sense, to a small degree, as far as religious beliefs go. But while the trolls view spirits as jealous or vengeful entities, the orcs look at them in an entirely different light. The orcs revere the spirits of their ancestors, who often stick around in the afterlife to offer advice or guidance. They believe the spirits of their ancestors can lend them their power. In fact, the orcs revere spirits in general -- the spirits of nature, of the elements, of the creatures around them. It's a very shamanistic way of looking at things, but the orcs were shaman at heart, before they were corrupted.
So to the orcs, the trolls' practices of sacrifice and cannibalism are ... disturbing, to put it mildly. Think of it in terms of a devout Catholic taking a stranger into their home and offering them a place to stay, because the stranger is terribly friendly and the two get along very, very well. Both believe in God and Christ, but where the Catholic faith has the tradition of drinking the blood of Christ in the form of wine at church, and both Christ and God are generally benevolent and loving beings, the stranger says that his faith has always dictated that the blood of Christ is something you use to take a bath with. This is so that Christ does not come and kill you in your sleep, because to the stranger, Christ is the harbinger of a fearful and vengeful God.
Right -- that just isn't going to go over very well. Both stranger and Catholic recognize God and Christ as existing, but the difference lies in the viewpoint.
Thrall asked that Vol'jin and the rest of the Darkspear stop practicing cannibalism and that they stopped sacrificing their enemies. Vol'jin agreed to this. It wasn't an immediate change, but over time, most trolls have stopped these practices. Cannibalism is a no-no, and as for sacrifices, they are made with animals rather than sentient beings. The fact of the matter was that while the Darkspears' beliefs were strong, their desire to survive was stronger. Add to this the fact that Sen'jin foresaw Thrall's leading his people to a brighter destiny -- and Vol'jin knew of this and wanted to honor his father's wishes.
If spirits you worshiped told you that a person was supposed to lead you to a brighter and better future, and that person suddenly came out and suggested that you drop the things that had been a part of your culture for so long ... well, there really would be only one clear choice to be made. End the darker practices and continue towards that path of greater destiny, or continue the practices and abandon the destiny the spirits have foretold. After all, the dark practices of voodoo and cannibalism hadn't exactly gotten the trolls very far. It's a bit of a catch-22 in a way, but the trolls chose to end the darker practices and instead try to adapt and embrace the ways of the orcs.
He loved power so much, in fact, that he let it overwhelm him completely and ignored the decree regarding the dark arts, instead choosing to use dark voodoo to rob members of the Darkspear tribe of their free will, forcing them to obey his every command. His army of mind-controlled Darkspear grew larger and larger each passing day, until Vol'jin and the few Darkspear left were forced to abandon the Echo Isles completely. They created Sen'jin Village, a small fishing community on the coast opposite the Echo Isles. Vol'jin left Gadrin in charge and made his way to Orgrimmar to serve as Thrall's advisor and to try and come up with a plan to retake the Echo Isles for good. Meanwhile, Master Gadrin was charged with doing something about Zalazane, something he tries to accomplish even now: sending players to kill his former apprentice and bring back his head.
While many appear to succeed and even return with Zalazane's "head," days later the trophies revert to their true forms -- coconuts or rocks painted to look like faces, or even the severed heads of former Darkspear. Just before Cataclysm, Vol'jin finally launches an effort to re-take Echo Isles for good and enlists the help of players to achieve this. Along the way, another surprise is discovered: Zalazane and his mind-controlled army aren't the only denizens of the Echo Isles. For years now, there has been a group of troll druids living on the isles, watching Zalazane's activities and waiting patiently for Vol'jin's return.
Regardless, the troll druids of the Echo Isles are very keen on helping Vol'jin take the islands back, and after the island has been restored, they are more than willing to stick around and teach the Darkspear the ways of the druidic arts. Most druids have some kind of deep connection with the earth, and it's likely that these trolls knew that something larger and more terrifying was approaching. The Darkspear would need all the help they could get.
More importantly, the trolls of the Echo Isles aren't terribly impressed with Garrosh, either. And the Horde that Garrosh leads isn't necessarily the Horde that Vol'jin or his people wish to be a part of. Suddenly, the respect shown to the Horde, the years of denying the cultural aspects of the trolls, the practices of dark voodoo ... well, it may not seem quite so necessary to hide these aspects of troll culture anymore. If Garrosh isn't going to give the trolls of the Darkspear the respect that they deserve, why should they bother altering practices to accommodate his beliefs?
The trolls of the Darkspear are not only learning the ways of the druid -- they are once again openly practicing the dark arts. Players will be able to roll troll warlocks in Cataclysm, and though there doesn't seem to be any explanation for their sudden appearance, the tense political situation between the Darkspear and the rest of the Horde suggests that perhaps the Darkspear simply isn't interested in dropping its beliefs for Garrosh. Not only that, but considering the scope of what the Horde faces in the upcoming expansion, it's going to need all the help it can get -- even if that help comes in the form of voodoo. As Vol'jin says after discussing the matter with Thrall, "The future right now be lookin' very grim and bloody." And he couldn't be more right.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW.com's Guide to Warcraft Lore.