Gumbercules is right. With the global cool down being what it is, what happens is: healer takes aggro with big cure, you provoke and have to wait for the GCD to be done to follow up with a high enmity attack but in that time the healer could have potentially taken aggro back with another big heal. Alos keep in mind HoT's and DoT's, when you taunt milliseconds later the HoT or DoT will trigger giving the other player aggro back. Simple fix: provoke doesn't trigger the GCD.