Sorin led Nissa to the Eye with the intent of sealing it once more, as Nahiri had done the first time the bonds were loosened. But Nissa had other ideas. She believed that, given their freedom, the Eldrazi would leave Zendikar and wreak their destruction on other planes, thus making her home safe again.

Why did Bolas release the Eldrazi?

To actively mobilize powerful planeswalkers in preparation to his summoning planeswalkers for his spark harvest.

What happened to Nissa Revane?

Nissa fell during the combat and awoke as a prisoner of the vampires. They told her that they were hunting the Mortifier, who Nissa mistakenly believed to be Anowon. They carried her for several days, all the while leaving Nissa wondering why they had left her alive before her allies rescued her.